Define Model Loss Function for Custom Training Loop

When you train a deep learning model with a custom training loop, the software minimizes the loss with respect to the learnable parameters. To minimize the loss, the software uses the gradients of the loss with respect to the learnable parameters. To calculate these gradients using automatic differentiation, you must define a model gradients function.

For an example showing how to train deep learning model with a dlnetwork object, see Train Network Using Custom Training Loop. For an example showing how to training a deep learning model defined as a function, see Train Network Using Model Function.

Create Model Loss Function for Model Defined as `dlnetwork` Object

If you have a deep learning model defined as a dlnetwork object, then create a model loss function that takes the dlnetwork object as input.

For a model specified as a dlnetwork object, create a function of the form [loss,gradients] = modelLoss(net,X,T), where net is the network, X is the network input, T contains the targets, and loss and gradients are the returned loss and gradients, respectively. Optionally, you can pass extra arguments to the gradients function (for example, if the loss function requires extra information), or return extra arguments (for example, the updated network state).

For example, this function returns the cross-entropy loss and the gradients of the loss with respect to the learnable parameters in the specified dlnetwork object net, given input data X, and targets T.

function [loss,gradients] = modelLoss(net,X,T)

    % Forward data through the dlnetwork object.
    Y = forward(net,X);

    % Compute loss.
    loss = crossentropy(Y,T);

    % Compute gradients.
    gradients = dlgradient(loss,net.Learnables);

end

Create Model Loss Function for Model Defined as Function

If you have a deep learning model defined as a function, then create a model loss function that takes the model learnable parameters as input.

For a model specified as a function, create a function of the form [loss,gradients] = modelLoss(parameters,X,T), where parameters contains the learnable parameters, X is the model input, T contains the targets, and loss and gradients are the returned loss and gradients, respectively. Optionally, you can pass extra arguments to the gradients function (for example, if the loss function requires extra information), or return extra arguments (for example, the updated model state).

For example, this function returns the cross-entropy loss and the gradients of the loss with respect to the learnable parameters parameters, given input data X, and targets T.

function [loss,gradients] = modelLoss(parameters,X,T)

    % Forward data through the model function.
    Y = model(parameters,X);

    % Compute loss.
    loss = crossentropy(Y,T);

    % Compute gradients.
    gradients = dlgradient(loss,parameters);

end

Evaluate Model Loss Function

To evaluate the model loss function using automatic differentiation, use the dlfeval function, which evaluates a function with automatic differentiation enabled. For the first input of dlfeval, pass the model loss function specified as a function handle. For the following inputs, pass the required variables for the model loss function. For the outputs of the dlfeval function, specify the same outputs as the model loss function.

For example, evaluate the model loss function modelLoss with a dlnetwork object net, input data X, and targets T, and return the model loss and gradients.

[loss,gradients] = dlfeval(@modelLoss,net,X,T);

Similarly, evaluate the model loss function modelLoss using a model function with learnable parameters specified by the structure parameters, input data X, and targets T, and return the model loss and gradients.

[loss,gradients] = dlfeval(@modelLoss,parameters,X,T);

Update Learnable Parameters Using Gradients

To update the learnable parameters, you can use these functions.

Function	Description
`adamupdate`	Update parameters using adaptive moment estimation (Adam)
`rmspropupdate`	Update parameters using root mean squared propagation (RMSProp)
`sgdmupdate`	Update parameters using stochastic gradient descent with momentum (SGDM)
`lbfgsupdate`	Update parameters using limited-memory BFGS (L-BFGS)
`dlupdate`	Update parameters using custom function

For example, update the learnable parameters of a dlnetwork object net using the adamupdate function.

[net,trailingAvg,trailingAvgSq] = adamupdate(net,gradients, ...
    trailingAvg,trailingAverageSq,iteration);

Here, gradients is the gradients of the loss with respect to the learnable parameters, and trailingAvg, trailingAvgSq, and iteration are the hyperparameters required by the adamupdate function.

Similarly, update the learnable parameters for a model function parameters using the adamupdate function.

[parameters,trailingAvg,trailingAvgSq] = adamupdate(parameters,gradients, ...
    trailingAvg,trailingAverageSq,iteration);

Use Model Loss Function in Custom Training Loop

When training a deep learning model using a custom training loop, evaluate the model loss and gradients and update the learnable parameters for each mini-batch.

This code snippet shows an example of using the dlfeval and adamupdate functions in a custom training loop.

iteration = 0;

% Loop over epochs.
for epoch = 1:numEpochs
    
    % Loop over mini-batches.
    for i = 1:numIterationsPerEpoch
        iteration = iteration + 1;

        % Prepare mini-batch.
        % ...

        % Evaluate model loss and gradients.
        [loss,gradients] = dlfeval(@modelLoss,net,X,T);

        % Update learnable parameters.
        [parameters,trailingAvg,trailingAvgSq] = adamupdate(parameters,gradients, ...
            trailingAvg,trailingAverageSq,iteration);

    end
end

For an example showing how to train a deep learning model with a dlnetwork object, see Train Network Using Custom Training Loop. For an example showing how to training a deep learning model defined as a function, see Train Network Using Model Function.

Debug Model Loss Functions

If the implementation of the model loss function has an issue, then the call to dlfeval can throw an error. Sometimes, when you use the dlfeval function, it is not clear which line of code is throwing the error. To help locate the error, you can try the following.

Call Model Loss Function Directly

Try calling the model loss function directly (that is, without using the dlfeval function) with generated inputs of the expected sizes. If any of the lines of code throw an error, then the error message provides extra detail. Note that when you do not use the dlfeval function, any calls to the dlgradient function throw an error.

% Generate image input data.
X = rand([28 28 1 100],'single');
X = dlarray(X);

% Generate one-hot encoded target data.
T = repmat(eye(10,'single'),[1 10]);

[loss,gradients] = modelLoss(net,X,T);

Run Model Loss Code Manually

Run the code inside the model loss function manually with generated inputs of the expected sizes and inspect the output and any thrown error messages.

For example, consider the following model loss function.

function [loss,gradients] = modelLoss(net,X,T)

    % Forward data through the dlnetwork object.
    Y = forward(net,X);

    % Compute loss.
    loss = crossentropy(Y,T);

    % Compute gradients.
    gradients = dlgradient(loss,net.Learnables);

end

Check the model loss function by running the following code.

% Generate image input data.
X = rand([28 28 1 100],'single');
X = dlarray(X);

% Generate one-hot encoded target data.
T = repmat(eye(10,'single'),[1 10]);

% Check forward pass.
Y = forward(net,X);

% Check loss calculation.
loss = crossentropy(Y,T)