## Define Model Gradients 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 Gradients Function for Model Defined as `dlnetwork` Object

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

For a model specified as a `dlnetwork` object, create a function of the form `gradients = modelGradients(dlnet,dlX,T)`, where `dlnet` is the network, `dlX` is the network input, `T` contains the targets, and `gradients` contains the returned gradients. 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, metrics for plotting the training progress).

For example, this function returns the gradients and the cross entropy loss for the specified `dlnetwork` object `dlnet`, input data `dlX`, and targets `T`.

```function [gradients, loss] = modelGradients(dlnet, dlX, T) % Forward data through the dlnetwork object. dlY = forward(dlnet,dlX); % Compute loss. loss = crossentropy(dlY,T); % Compute gradients. gradients = dlgradient(loss,dlnet); end```

### Create Model Gradients Function for Model Defined as Function

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

For a model specified as a function, create a function of the form `gradients = modelGradients(parameters,dlX,T)`, where `parameters` contains the learnable parameters, `dlX` is the model input, `T` contains the targets, and `gradients` contains the returned gradients. 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, metrics for plotting the training progress).

For example, this function returns the gradients and the cross-entropy loss for the deep learning model function `model` with the specified learnable parameters `parameters`, input data `dlX`, and targets `T`.

```function [gradients, loss] = modelGradients(parameters, dlX, T) % Forward data through the model function. dlY = model(parameters,dlX); % Compute loss. loss = crossentropy(dlY,T); % Compute gradients. gradients = dlgradient(loss,parameters); end```

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

For example, evaluate the model gradients function `modelGradients` with a `dlnetwork` object `dlnet`, input data `dlX`, and targets `T`, and return the model gradients and loss.

`[gradients, loss] = dlfeval(@modelGradients,dlnet,dlX,T);`

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

`[gradients, loss] = dlfeval(@modelGradients,parameters,dlX,T);`

### Update Learnable Parameters Using Gradients

To update the learnable parameters using the gradients, you can use the following functions.

FunctionDescription
`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)
`dlupdate`Update parameters using custom function

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

```[dlnet,trailingAvg,trailingAvgSq] = adamupdate(dlnet,gradients, ... trailingAvg,trailingAverageSq,iteration);```
`gradients` is the output of the model gradients function, 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);```
`gradients` is the output of the model gradients function, and `trailingAvg`, `trailingAvgSq`, and `iteration` are the hyperparameters required by the `adamupdate` function.

### Use Model Gradients Function in Custom Training Loop

When training a deep learning model using a custom training loop, evaluate the model 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 gradients. [gradients, loss] = dlfeval(@modelGradients,dlnet,dlX,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.

If the implementation of the model gradients 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 Gradients Function Directly

Try calling the model gradients 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'); dlX = dlarray(dlX); % Generate one-hot encoded target data. T = repmat(eye(10,'single'),[1 10]); [gradients, loss] = modelGradients(dlnet,dlX,T);```

#### Run Model Gradients Code Manually

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

For example, consider the following model gradients function.

```function [gradients, loss] = modelGradients(dlnet, dlX, T) % Forward data through the dlnetwork object. dlY = forward(dlnet,dlX); % Compute loss. loss = crossentropy(dlY,T); % Compute gradients. gradients = dlgradient(loss,dlnet); end```

Check the model gradients function by running the following code.

```% Generate image input data. X = rand([28 28 1 100],'single'); dlX = dlarray(dlX); % Generate one-hot encoded target data. T = repmat(eye(10,'single'),[1 10]); % Check forward pass. dlY = forward(dlnet,dlX); % Check loss calculation. loss = crossentropy(dlX,T)```