Check Custom Layer Validity

If you create a custom deep learning layer, then you can use the checkLayer function to check that the layer is valid. The function checks layers for validity, GPU compatibility, correctly defined gradients, and code generation compatibility. To check that a layer is valid, run the following command:

checkLayer(layer,validInputSize,'ObservationDimension',dim)

where layer is an instance of the layer, validInputSize is a vector or cell array specifying the valid input sizes to the layer, and dim specifies the dimension of the observations in the layer input data. For large input sizes, the gradient checks take longer to run. To speed up the tests, specify a smaller valid input size.

Check Layer Validity

Open Live Script

Check the validity of the example custom layer preluLayer.

Define a custom PReLU layer. To create this layer, save the file preluLayer.m in the current folder.

Create an instance of the layer and check that it is valid using checkLayer. Set the valid input size to the typical size of a single observation input to the layer. For a single input, the layer expects observations of size h-by-w-by-c, where h, w, and c are the height, width, and number of channels of the previous layer output, respectively.

Specify validInputSize as the typical size of an input array.

layer = preluLayer(20,'prelu');
validInputSize = [5 5 20];
checkLayer(layer,validInputSize)

Skipping multi-observation tests. To enable tests with multiple observations, specify the 'ObservationDimension' option.
For 2-D image data, set 'ObservationDimension' to 4.
For 3-D image data, set 'ObservationDimension' to 5.
For sequence data, set 'ObservationDimension' to 2.
 
Skipping GPU tests. No compatible GPU device found.
 
Skipping code generation compatibility tests. To check validity of the layer for code generation, specify the 'CheckCodegenCompatibility' and 'ObservationDimension' options.
 
Running nnet.checklayer.TestLayerWithoutBackward
.........
Done nnet.checklayer.TestLayerWithoutBackward
__________

Test Summary:
	 9 Passed, 0 Failed, 0 Incomplete, 12 Skipped.
	 Time elapsed: 0.11901 seconds.

The results show the number of passed, failed, and skipped tests. If you do not specify the 'ObservationsDimension' option, or do not have a GPU, then the function skips the corresponding tests.

Check Multiple Observations

For multi-observation input, the layer expects an array of observations of size h-by-w-by-c-by-N, where h, w, and c are the height, width, and number of channels, respectively, and N is the number of observations.

To check the layer validity for multiple observations, specify the typical size of an observation and set 'ObservationDimension' to 4.

layer = preluLayer(20,'prelu');
validInputSize = [5 5 20];
checkLayer(layer,validInputSize,'ObservationDimension',4)

Skipping GPU tests. No compatible GPU device found.
 
Skipping code generation compatibility tests. To check validity of the layer for code generation, specify the 'CheckCodegenCompatibility' and 'ObservationDimension' options.
 
Running nnet.checklayer.TestLayerWithoutBackward
.......... ...
Done nnet.checklayer.TestLayerWithoutBackward
__________

Test Summary:
	 13 Passed, 0 Failed, 0 Incomplete, 8 Skipped.
	 Time elapsed: 0.069311 seconds.

In this case, the function does not detect any issues with the layer.

List of Tests

The checkLayer function checks the validity of a custom layer by performing a series of tests.

Intermediate Layers

The checkLayer function uses these tests to check the validity of custom intermediate layers (layers of type nnet.layer.Layer).

Test	Description
`functionSyntaxesAreCorrect`	The syntaxes of the layer functions are correctly defined.
`predictDoesNotError`	`predict` does not error.
`forwardDoesNotError`	When specified, `forward` does not error.
`forwardPredictAreConsistentInSize`	When `forward` is specified, `forward` and `predict` output values of the same size.
`backwardDoesNotError`	When specified, `backward` does not error.
`backwardIsConsistentInSize`	When `backward` is specified, the outputs of `backward` are consistent in size: The derivatives with respect to each input are the same size as the corresponding input. The derivatives with respect to each learnable parameter are the same size as the corresponding learnable parameter.
`predictIsConsistentInType`	The outputs of `predict` are consistent in type with the inputs.
`forwardIsConsistentInType`	When `forward` is specified, the outputs of `forward` are consistent in type with the inputs.
`backwardIsConsistentInType`	When `backward` is specified, the outputs of `backward` are consistent in type with the inputs.
`gradientsAreNumericallyCorrect`	When `backward` is specified, the gradients computed in `backward` are consistent with the numerical gradients.
`backwardPropagationDoesNotError`	When `backward` is not specified, the derivatives can be computed using automatic differentiation.
`codegenPragmaDefinedInClassDef`	The pragma `"%#codegen"` for code generation is specified in class file.
`checkForSupportedLayerPropertiesForCodegen`	The layer properties support code generation.
`predictIsValidForCodeGeneration`	`predict` is valid for code generation.

The tests predictIsConsistentInType, forwardIsConsistentInType, and backwardIsConsistentInType also check for GPU compatibility. To execute the layer functions on a GPU, the functions must support inputs and outputs of type gpuArray with the underlying data type single.

Output Layers

The checkLayer function uses these tests to check the validity of custom output layers (layers of type nnet.layer.ClassificationLayer or nnet.layer.RegressionLayer).

Test	Description
`forwardLossDoesNotError`	`forwardLoss` does not error.
`backwardLossDoesNotError`	`backwardLoss` does not error.
`forwardLossIsScalar`	The output of `forwardLoss` is scalar.
`backwardLossIsConsistentInSize`	When `backwardLoss` is specified, the output of `backwardLoss` is consistent in size: `dLdY` is the same size as the predictions `Y`.
`forwardLossIsConsistentInType`	The output of `forwardLoss` is consistent in type: `loss` is the same type as the predictions `Y`.
`backwardLossIsConsistentInType`	When `backwardLoss` is specified, the output of `backwardLoss` is consistent in type: `dLdY` must be the same type as the predictions `Y`.
`gradientsAreNumericallyCorrect`	When `backwardLoss` is specified, the gradients computed in `backwardLoss` are numerically correct.
`backwardPropagationDoesNotError`	When `backwardLoss` is not specified, the derivatives can be computed using automatic differentiation.

The forwardLossIsConsistentInType and backwardLossIsConsistentInType tests also check for GPU compatibility. To execute the layer functions on a GPU, the functions must support inputs and outputs of type gpuArray with the underlying data type single.

Generated Data

To check the layer validity, the checkLayer function generates data depending on the type of layer:

Layer Type Description of Generated Data

Intermediate Values in the range [-1,1]

Regression output Predictions and targets with values in the range [-1,1]

Classification output

Layer Type	Description of Generated Data
Intermediate	Values in the range [-1,1]
Regression output	Predictions and targets with values in the range [-1,1]
Classification output	Predictions with values in the range [0,1]. If you specify the `'ObservationDimension'` option, then the targets are one-hot encoded vectors (vectors containing a single 1, and 0 elsewhere). If you do not specify the `'ObservationDimension'` option, then the targets are values in the range [0,1].

Predictions with values in the range [0,1].

If you specify the 'ObservationDimension' option, then the targets are one-hot encoded vectors (vectors containing a single 1, and 0 elsewhere).

If you do not specify the 'ObservationDimension' option, then the targets are values in the range [0,1].

To check for multiple observations, specify the observation dimension using the 'ObservationDimension' name-value pair. If you specify the observation dimension, then the checkLayer function checks that the layer functions are valid using generated data with mini-batches of size 1 and 2. If you do not specify this name-value pair, then the function skips the tests that check that the layer functions are valid for multiple observations.

Diagnostics

If a test fails when you use checkLayer, then the function provides a test diagnostic and a framework diagnostic. The test diagnostic highlights any issues found with the layer. The framework diagnostic provides more detailed information.

Function Syntaxes

The test functionSyntaxesAreCorrect checks that the layer functions have correctly defined syntaxes.

Test Diagnostic	Description	Possible Solution
`Incorrect number of input arguments for 'predict' in Layer`.	The syntax for the `predict` function is not consistent with the number of layer inputs.	Specify the correct number of input and output arguments in `predict`. The syntax for `predict` is [Z1,…,Zm] = predict(layer,X1,…,Xn) where `X1,…,Xn` are the `n` layer inputs and `Z1,…,Zm` are the `m` layer outputs. The values `n` and `m` must correspond to the `NumInputs` and `NumOutputs` properties of the layer. Tip If the number of inputs to `predict` can vary, then use `varargin` instead of `X1,…,Xn`. In this case, `varargin` is a cell array of the inputs, where `varargin{i}` corresponds to `Xi`. If the number of outputs can vary, then use `varargout` instead of `Z1,…,Zm`. In this case, `varargout` is a cell array of the outputs, where `varargout{j}` corresponds to `Zj`. Tip If the custom layer has a `dlnetwork` object for a learnable parameter, then in the `predict` function of the custom layer, use the `predict` function for the `dlnetwork`. Using the `dlnetwork` object `predict` function ensures that the software uses the correct layer operations for prediction.
`Incorrect number of output arguments for 'predict' in Layer`	The syntax for the `predict` function is not consistent with the number of layer outputs.
`Incorrect number of input arguments for 'forward' in Layer`	The syntax for the optional `forward` function is not consistent with the number of layer inputs.	Specify the correct number of input and output arguments in `forward`. The syntax for `forward` is [Z1,…,Zm,memory] = forward(layer,X1,…,Xn) where `X1,…,Xn` are the `n` layer inputs, `Z1,…,Zm` are the `m` layer outputs, and `memory` is the memory of the layer. Tip If the number of inputs to `forward` can vary, then use `varargin` instead of `X1,…,Xn`. In this case, `varargin` is a cell array of the inputs, where `varargin{i}` corresponds to `Xi`. If the number of outputs can vary, then use `varargout` instead of `Z1,…,Zm`. In this case, `varargout` is a cell array of the outputs, where `varargout{j}` corresponds to `Zj` for `j`=1,…,`NumOutputs` and `varargout{NumOutputs+1}` corresponds to `memory`. Tip If the custom layer has a `dlnetwork` object for a learnable parameter, then in the `forward` function of the custom layer, use the `forward` function of the `dlnetwork` object. Using the `dlnetwork` object `forward` function ensures that the software uses the correct layer operations for training.
`Incorrect number of output arguments for 'forward' in Layer`	The syntax for the optional `forward` function is not consistent with the number of layer outputs.
`Incorrect number of input arguments for 'backward' in Layer`	The syntax for the optional `backward` function is not consistent with the number of layer inputs and outputs.	Specify the correct number of input and output arguments in `backward`. The syntax for `backward` is [dLdX1,…,dLdXn,dLdW1,…,dLdWk] = backward(layer,X1,…,Xn,Z1,…,Zm,dLdZ1,…,dLdZm,memory) where: `X1,…,Xn` are the `n` layer inputs `Z1,…,Zm` are the `m` outputs of the layer forward functions `dLdZ1,…,dLdZm` are the gradients backward propagated from the next layer `memory` is the memory output of `forward` if `forward` is defined, otherwise, `memory` is `[]`. For the outputs, `dLdX1,…,dLdXn` are the derivatives of the loss with respect to the layer inputs and `dLdW1,…,dLdWk` are the derivatives of the loss with respect to the `k` learnable parameters. To reduce memory usage by preventing unused variables being saved between the forward and backward pass, replace the corresponding input arguments with `~`. Tip If the number of inputs to `backward` can vary, then use `varargin` instead of the input arguments after `layer`. In this case, `varargin` is a cell array of the inputs, where `varargin{i}` corresponds to `Xi` for `i`=1,…,`NumInputs`, `varargin{NumInputs+j}` and `varargin{NumInputs+NumOutputs+j}` correspond to `Zj` and `dLdZj`, respectively, for `j`=1,…,`NumOutputs`, and `varargin{end}` corresponds to `memory`. If the number of outputs can vary, then use `varargout` instead of the output arguments. In this case, `varargout` is a cell array of the outputs, where `varargout{i}` corresponds to `dLdXi` for `i`=1,…,`NumInputs` and `varargout{NumInputs+t}` corresponds to `dLdWt` for `t`=1,…,`k`, where `k` is the number of learnable parameters. Tip If the layer forward functions support `dlarray` objects, then the software automatically determines the backward function and you do not need to specify the `backward` function. For a list of functions that support `dlarray` objects, see List of Functions with dlarray Support.
`Incorrect number of output arguments for 'backward' in Layer`	The syntax for the optional `backward` function is not consistent with the number of layer outputs.

For layers with multiple inputs or outputs, you must set the values of the layer properties NumInputs (or alternatively, InputNames) and NumOutputs (or alternatively, OutputNames) in the layer constructor function, respectively.

Multiple Observations

The checkLayer function checks that the layer functions are valid for single and multiple observations. To check for multiple observations, specify the observation dimension using the 'ObservationDimension' name-value pair. If you specify the observation dimension, then the checkLayer function checks that the layer functions are valid using generated data with mini-batches of size 1 and 2. If you do not specify this name-value pair, then the function skips the tests that check that the layer functions are valid for multiple observations.

Test Diagnostic Description Possible Solution

Test Diagnostic	Description	Possible Solution
`Skipping multi-observation tests. To enable checks with multiple observations, specify the 'ObservationDimension' parameter in checkLayer`.	If you do not specify the `'ObservationDimension'` parameter in `checkLayer`, then the function skips the tests that check data with multiple observations.	Use the command `checkLayer(layer,validInputSize,'ObservationDimension',dim)`, where `layer` is an instance of the custom layer, `validInputSize` is a vector specifying the valid input size to the layer, and `dim` specifies the dimension of the observations in the layer input. For more information, see Layer Input Sizes.

Skipping multi-observation tests. To enable checks with
                                    multiple observations, specify the 'ObservationDimension'
                                    parameter in checkLayer

If you do not specify the 'ObservationDimension' parameter in checkLayer, then the function skips the tests that check data with multiple observations.

Use the command checkLayer(layer,validInputSize,'ObservationDimension',dim), where layer is an instance of the custom layer, validInputSize is a vector specifying the valid input size to the layer, and dim specifies the dimension of the observations in the layer input.

For more information, see Layer Input Sizes.

Functions Do Not Error

These tests check that the layers do not error when passed input data of valid size.

Intermediate Layers. The tests predictDoesNotError, forwardDoesNotError, and backwardDoesNotError check that the layer functions do not error when passed inputs of valid size. If you specify an observation dimension, then the function checks the layer for both a single observation and multiple observations.

Test Diagnostic	Description	Possible Solution
`The function 'predict' threw an error:`	The `predict` function errors when passed data of size `validInputSize`.	Address the error described in the `Framework Diagnostic` section. Tip If the layer forward functions support `dlarray` objects, then the software automatically determines the backward function and you do not need to specify the `backward` function. For a list of functions that support `dlarray` objects, see List of Functions with dlarray Support.
`The function 'forward' threw an error:`	The optional `forward` function errors when passed data of size `validInputSize`.
`The function 'backward' threw an error:`	The optional `backward` function errors when passed the output of `predict`.

Output Layers. The tests forwardLossDoesNotError and backwardLossDoesNotError check that the layer functions do not error when passed inputs of valid size. If you specify an observation dimension, then the function checks the layer for both a single observation and multiple observations.

Test Diagnostic	Description	Possible Solution
`The function 'forwardLoss' threw an error:`	The `forwardLoss` function errors when passed data of size `validInputSize`.	Address the error described in the `Framework Diagnostic` section. Tip If the `forwardLoss` function supports `dlarray` objects, then the software automatically determines the backward loss function and you do not need to specify the `backwardLoss` function. For a list of functions that support `dlarray` objects, see List of Functions with dlarray Support.
`The function 'backwardLoss' threw an error:`	The optional `backwardLoss` function errors when passed data of size `validInputSize`.

Outputs Are Consistent in Size

These tests check that the layer function outputs are consistent in size.

Intermediate Layers. The test backwardIsConsistentInSize checks that the backward function outputs derivatives of the correct size.

The syntax for backward is

[dLdX1,…,dLdXn,dLdW1,…,dLdWk] = backward(layer,X1,…,Xn,Z1,…,Zm,dLdZ1,…,dLdZm,memory)

where:

X1,…,Xn are the n layer inputs
Z1,…,Zm are the m outputs of the layer forward functions
dLdZ1,…,dLdZm are the gradients backward propagated from the next layer
memory is the memory output of forward if forward is defined, otherwise, memory is [].

For the outputs, dLdX1,…,dLdXn are the derivatives of the loss with respect to the layer inputs and dLdW1,…,dLdWk are the derivatives of the loss with respect to the k learnable parameters. To reduce memory usage by preventing unused variables being saved between the forward and backward pass, replace the corresponding input arguments with ~.

Tip

If the number of inputs to backward can vary, then use varargin instead of the input arguments after layer. In this case, varargin is a cell array of the inputs, where varargin{i} corresponds to Xi for i=1,…,NumInputs, varargin{NumInputs+j} and varargin{NumInputs+NumOutputs+j} correspond to Zj and dLdZj, respectively, for j=1,…,NumOutputs, and varargin{end} corresponds to memory.

If the number of outputs can vary, then use varargout instead of the output arguments. In this case, varargout is a cell array of the outputs, where varargout{i} corresponds to dLdXi for i=1,…,NumInputs and varargout{NumInputs+t} corresponds to dLdWt for t=1,…,k, where k is the number of learnable parameters.

The derivatives dLdX1, …, dLdXn must be the same size as the corresponding layer inputs, and dLdW1,…,dLdWk must be the same size as the corresponding learnable parameters. The sizes must be consistent for input data with single and multiple observations.

Test Diagnostic	Description	Possible Solution
`Incorrect size of 'dLdX' for 'backward'`.	The derivatives of the loss with respect to the layer inputs must be the same size as the corresponding layer input.	Return the derivatives `dLdX1,…,dLdXn` with the same size as the corresponding layer inputs `X1,…,Xn`.
`Incorrect size of the derivative of the loss with respect to the input 'in1' for 'backward'`
`The size of 'Z' returned from 'forward' must be the same as for 'predict'`.	The outputs of `predict` must be the same size as the corresponding outputs of `forward`.	Return the outputs `Z1,…,Zm` of `predict` with the same size as the corresponding outputs `Z1,…,Zm` of `forward`.
`Incorrect size of the derivative of the loss with respect to 'W' for 'backward'`.	The derivatives of the loss with respect to the learnable parameters must be the same size as the corresponding learnable parameters.	Return the derivatives `dLdW1,…,dLdWk` with the same size as the corresponding learnable parameters `W1,…,Wk`.

Tip

If the layer forward functions support dlarray objects, then the software automatically determines the backward function and you do not need to specify the backward function. For a list of functions that support dlarray objects, see List of Functions with dlarray Support.

Output Layers. The test forwardLossIsScalar checks that the output of the forwardLoss function is scalar. When the backwardLoss function is specified, the test backwardLossIsConsistentInSize checks that the outputs of forwardLoss and backwardLoss are of the correct size.

The syntax for forwardLoss is loss = forwardLoss(layer, Y, T). The input Y corresponds to the predictions made by the network. These predictions are the output of the previous layer. The input T corresponds to the training targets. The output loss is the loss between Y and T according to the specified loss function. The output loss must be scalar.

If the forwardLoss function supports dlarray objects, then the software automatically determines the backward loss function and you do not need to specify the backwardLoss function. For a list of functions that support dlarray objects, see List of Functions with dlarray Support.

The syntax for backwardLoss is dLdY = backwardLoss(layer, Y, T). The input Y contains the predictions made by the network and T contains the training targets. The output dLdY is the derivative of the loss with respect to the predictions Y. The output dLdY must be the same size as the layer input Y.

Test Diagnostic Description Possible Solution

Test Diagnostic	Description	Possible Solution
`Incorrect size of 'loss' for 'forwardLoss'`.	The output `loss` of `forwardLoss` must be a scalar.	Return the output `loss` as a scalar. For example, if you have multiple values of the loss, then you can use `mean` or `sum`.
`Incorrect size of the derivative of loss 'dLdY' for 'backwardLoss'`.	When `backwardLoss` is specified, the derivatives of the loss with respect to the layer input must be the same size as the layer input.	Return derivative `dLdY` with the same size as the layer input `Y`. If the `forwardLoss` function supports `dlarray` objects, then the software automatically determines the backward loss function and you do not need to specify the `backwardLoss` function. For a list of functions that support `dlarray` objects, see List of Functions with dlarray Support.

Incorrect size of 'loss' for
                                            'forwardLoss'

The output loss of forwardLoss must be a scalar.

Return the output loss as a scalar. For example, if you have multiple values of the loss, then you can use mean or sum.

Incorrect size of the derivative of loss 'dLdY'
                                            for 'backwardLoss'

When backwardLoss is specified, the derivatives of the loss with respect to the layer input must be the same size as the layer input.

Return derivative dLdY with the same size as the layer input Y.

Consistent Data Types and GPU Compatibility

These tests check that the layer function outputs are consistent in type and that the layer functions are GPU compatible.

If the layer forward functions fully support dlarray objects, then the layer is GPU compatible. Otherwise, to be GPU compatible, the layer functions must support inputs and return outputs of type gpuArray (Parallel Computing Toolbox).

Many MATLAB^® built-in functions support gpuArray (Parallel Computing Toolbox) and dlarray input arguments. For a list of functions that support dlarray objects, see List of Functions with dlarray Support. For a list of functions that execute on a GPU, see Run MATLAB Functions on a GPU (Parallel Computing Toolbox). To use a GPU for deep learning, you must also have a CUDA^® enabled NVIDIA^® GPU with compute capability 3.0 or higher. For more information on working with GPUs in MATLAB, see GPU Computing in MATLAB (Parallel Computing Toolbox).

Intermediate Layers. The tests predictIsConsistentInType, forwardIsConsistentInType, and backwardIsConsistentInType check that the layer functions output variables of the correct data type. The tests check that the layer functions return consistent data types when given inputs of the data types single, double, and gpuArray with the underlying types single or double.

Tip

If you preallocate arrays using functions like zeros, then you must ensure that the data types of these arrays are consistent with the layer function inputs. To create an array of zeros of the same data type of another array, use the 'like' option of zeros. For example, to initialize an array of zeros of size sz with the same data type as the array X, use Z = zeros(sz,'like',X).

Test Diagnostic	Description	Possible Solution
`Incorrect type of 'Z' for 'predict'`.	The types of the outputs `Z1,…,Zm` of the `predict` function must be consistent with the inputs `X1,…,Xn`.	Return the outputs `Z1,…,Zm` with the same type as the inputs `X1,…,Xn`.
`Incorrect type of output 'out1' for 'predict'`.
`Incorrect type of 'Z' for 'forward'`.	The types of the outputs `Z1,…,Zm` of the optional `forward` function must be consistent with the inputs `X1,…,Xn`.
`Incorrect type of output 'out1' for 'forward'`.
`Incorrect type of 'dLdX' for 'backward'`.	The types of the derivatives `dLdX1,…,dLdXn` of the optional `backward` function must be consistent with the inputs `X1,…,Xn`.	Return the derivatives `dLdX1,…,dLdXn` with the same type as the inputs `X1,…,Xn`.
`Incorrect type of the derivative of the loss with respect to the input 'in1' for 'backward'`.
`Incorrect type of the derivative of loss with respect to 'W' for 'backward'`.	The type of the derivative of the loss of the learnable parameters must be consistent with the corresponding learnable parameters.	For each learnable parameter, return the derivative with the same type as the corresponding learnable parameter.

Tip

Output Layers. The tests forwardLossIsConsistentInType and backwardLossIsConsistentInType check that the layer functions output variables of the correct data type. The tests check that the layers return consistent data types when given inputs of the data types single, double, and gpuArray with the underlying types single or double.

Test Diagnostic	Description	Possible Solution
`Incorrect type of 'loss' for 'forwardLoss'`.	The type of the output `loss` of the `forwardLoss` function must be consistent with the input `Y`.	Return `loss` with the same type as the input `Y`.
`Incorrect type of the derivative of loss 'dLdY' for 'backwardLoss'`.	The type of the output `dLdY` of the optional `backwardLoss` function must be consistent with the input `Y`.	Return `dLdY` with the same type as the input `Y`.

Tip

Correct Gradients

The test gradientsAreNumericallyCorrect checks that the gradients computed by the layer functions are numerically correct. The test backwardPropagationDoesNotError checks that the derivatives can be computed using automatic differentiation.

Intermediate Layers. When the optional backward function is not specified, the test backwardPropagationDoesNotError checks that the derivatives can be computed using automatic differentiation. When the optional backward function is specified, the test gradientsAreNumericallyCorrect tests that the gradients computed in backward are numerically correct.

Test Diagnostic	Description	Possible Solution
`Expected a dlarray with no dimension labels, but instead found labels`.	When the optional `backward` function is not specified, the layer forward functions must output `dlarray` objects without dimension labels.	Ensure that any `dlarray` objects created in the layer forward functions do not contain dimension labels.
`Unable to backward propagate through the layer. Check that the 'forward' function fully supports automatic differentiation. Alternatively, implement the 'backward' function manually`.	One or more of the following: When the optional `backward` function is not specified, the layer forward functions do not support `dlarray` objects. When the optional `backward` function is not specified, the tracing of the input `dlarray` objects in the forward functions have been broken. For example, by using the `extractdata` function.	Check that the forward functions support `dlarray` objects. For a list of functions that support `dlarray` objects, see List of Functions with dlarray Support. Check that the derivatives of the input `dlarray` objects can be traced. To learn more about the derivative trace of `dlarray` objects, see Derivative Trace. Alternatively, define a custom backward function by creating a function named `backward`. To learn more, see Backward Function.
`Unable to backward propagate through the layer. Check that the 'predict' function fully supports automatic differentiation. Alternatively, implement the 'backward' function manually`.
`The derivative 'dLdX' for 'backward' is inconsistent with the numerical gradient`.	One or more of the following: When the optional `backward` function is specified, the derivative is incorrectly computed The forward functions are non-differentiable at some input points Error tolerance is too small	If the layer forward functions support `dlarray` objects, then the software automatically determines the backward function and you can omit the backward function. For a list of functions that support `dlarray` objects, see List of Functions with dlarray Support. Check that the derivatives in `backward` are correctly computed. If the derivatives are correctly computed, then in the `Framework Diagnostic` section, manually check the absolute and relative error between the actual and expected values of the derivative. If the absolute and relative errors are within an acceptable margin of the tolerance, then you can ignore this test diagnostic.
`The derivative of the loss with respect to the input 'in1' for 'backward' is inconsistent with the numerical gradient`.
`The derivative of loss with respect to 'W' for 'backward' is inconsistent with the numerical gradient`.

Tip

Output Layers. When the optional backwardLoss function is not specified, the test backwardPropagationDoesNotError checks that the derivatives can be computed using automatic differentiation. When the optional backwardLoss function is specified, the test gradientsAreNumericallyCorrect tests that the gradients computed in backwardLoss are numerically correct.

Test Diagnostic Description Possible Solution

Expected a dlarray with no dimension labels, but instead found labels When the optional backwardLoss function is not specified, the forwardLoss function must output dlarray objects without dimension labels. Ensure that any dlarray objects created in the forwardLoss function does not contain dimension labels.

Test Diagnostic	Description	Possible Solution
`Expected a dlarray with no dimension labels, but instead found labels`	When the optional `backwardLoss` function is not specified, the `forwardLoss` function must output `dlarray` objects without dimension labels.	Ensure that any `dlarray` objects created in the `forwardLoss` function does not contain dimension labels.
`Unable to backward propagate through the layer. Check that the 'forwardLoss' function fully supports automatic differentiation. Alternatively, implement the 'backwardLoss' function manually`	One or more of the following: When the optional `backwardLoss` function is not specified, the layer `forwardLoss` function does not support `dlarray` objects. When the optional `backwardLoss` function is not specified, the tracing of the input `dlarray` objects in the `forwardLoss` function has been broken. For example, by using the `extractdata` function.	Check that the `forwardLoss` function supports `dlarray` objects. For a list of functions that support `dlarray` objects, see List of Functions with dlarray Support. Check that the derivatives of the input `dlarray` objects can be traced. To learn more about the derivative trace of `dlarray` objects, see Derivative Trace. Alternatively, define a custom backward loss function by creating a function named `backwardLoss`. To learn more, see Loss Functions.
`The derivative 'dLdY' for 'backwardLoss' is inconsistent with the numerical gradient`.	One or more of the following: The derivative with respect to the predictions `Y` is incorrectly computed Function is non-differentiable at some input points Error tolerance is too small	Check that the derivatives in `backwardLoss` are correctly computed. If the derivatives are correctly computed, then in the `Framework Diagnostic` section, manually check the absolute and relative error between the actual and expected values of the derivative. If the absolute and relative errors are within an acceptable margin of the tolerance, then you can ignore this test diagnostic.

Unable to backward propagate through the layer.
                                            Check that the 'forwardLoss' function fully supports
                                            automatic differentiation. Alternatively, implement the
                                            'backwardLoss' function manually

One or more of the following:

When the optional backwardLoss function is not specified, the layer forwardLoss function does not support dlarray objects.
When the optional backwardLoss function is not specified, the tracing of the input dlarray objects in the forwardLoss function has been broken. For example, by using the extractdata function.

Check that the forwardLoss function supports dlarray objects. For a list of functions that support dlarray objects, see List of Functions with dlarray Support.

Check that the derivatives of the input dlarray objects can be traced. To learn more about the derivative trace of dlarray objects, see Derivative Trace.

Alternatively, define a custom backward loss function by creating a function named backwardLoss. To learn more, see Loss Functions.

The derivative 'dLdY' for 'backwardLoss' is
                                            inconsistent with the numerical
                                        gradient

One or more of the following:

The derivative with respect to the predictions Y is incorrectly computed
Function is non-differentiable at some input points
Error tolerance is too small

Check that the derivatives in backwardLoss are correctly computed.

If the derivatives are correctly computed, then in the Framework Diagnostic section, manually check the absolute and relative error between the actual and expected values of the derivative.

If the absolute and relative errors are within an acceptable margin of the tolerance, then you can ignore this test diagnostic.

Tip

Code Generation Compatibility

If you set the 'CheckCodegenCompatibility' option to true, then the checkLayer function checks the layer for code generation compatibility.

The test codegenPragmaDefinedInClassDef checks that the layer definition contains the code generation pragma %#codegen. The test checkForSupportedLayerPropertiesForCodegen checks that the layer properties support code generation. The test predictIsValidForCodegeneration checks that the outputs of predict are consistent in dimension and batch size.

Code generation supports intermediate layers with 2-D image input only.

Test Diagnostic	Description	Possible Solution
`Specify '%#codegen' in the class definition of custom layer`	The layer definition does not include the pragma `"%#codegen"` for code generation.	Add the `%#codegen` directive (or pragma) to your layer definition to indicate that you intend to generate code for this layer. Adding this directive instructs the MATLAB Code Analyzer to help you diagnose and fix violations that would result in errors during code generation.
`Nonscalar layer properties must be type single or double or character array for custom layer`	The layer contains non-scalar properties of type other than single, double, or character array.	Convert non-scalar properties to use a representation of type single, double, or character array. For example, convert a categorical array to an array of integers of type `double` representing the categories.
`Scalar layer properties must be numeric, logical, or string for custom layer`	The layer contains scalar properties of type other than numeric, logical, or string.	Convert scalar properties to use a numeric representation, or a representation of type logical or string. For example, convert a categorical scalar to an integer of type `double` representing the category.
`For code generation, 'Z' must have the same number of dimensions as the layer input`.	The number of dimensions of the output `Z` of `predict` does not match the number of dimensions of the layer inputs.	In the `predict` function, return the outputs with the same number of dimensions as the layer inputs.
`For code generation, 'Z' must have the same batch size as the layer input`.	The size of the batch size of the output `Z` of `predict` does not match the size of the batch size of the layer inputs.	In the `predict` function, return the outputs with the batch size as the layer inputs.

Documentation