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Tutorial

Extract the AutoNN files somewhere, then pull up a Matlab console and get confortable! The first step is to add MatConvNet to the path (with vl_setupnn), as well as AutoNN (with setup_autonn). Note that no compilation is required for AutoNN.

Defining networks

A deep neural network is a particular case of a computational graph. With AutoNN, this graph is created by composing overloaded operators of special objects, much like in other modern frameworks that shall not be named. We start by defining one of the network's inputs:

images = Input()

We can then define the first operation, a convolution with a given kernel shape, which in MatConvNet is computed by the vl_nnconv function:

conv1 = vl_nnconv(images, 'size', [5, 5, 1, 20])

The resulting object has class Layer. The Input that we created earlier also subclasses Layer. All the MatConvNet functions are overloaded by the Layer class, so that instead of running them immediately, they will produce a new Layer object. It is this nested structure of Layer objects that defines the network's topology.

According to the deep learning mantra, we obviously need to add more layers. For demonstration purposes, we'll add just one pooling layer and another convolution:

pool1 = vl_nnpool(conv1, 2, 'stride', 2);
conv2 = vl_nnconv(pool1, 'size', [5, 5, 20, 10]);

The syntax for vl_nnconv shown above creates parameters for filters and biases automatically, initialized with the Xavier method (see help Layer.vl_nnconv for other initialization options).

We could, of course, also create these parameters explicitly. They are objects of class Param (again, a subclass of Layer), and on creation we specify their initial value:

filters = Param('value', 0.01 * randn(5, 5, 1, 20, 'single'));
biases = Param('value', zeros(20, 1, 'single'));

Our conv1 layer could then, alternatively, be defined as:

conv1_alt = vl_nnconv(images, filters, biases);

This follows the function signature for MatConvNet's vl_nnconv function exactly (which can be checked with help vl_nnconv). The difference is that, instead of calling vl_nnconv immediately, the function call's signature is stored in a new Layer object, for later evaluation.

Note also that the filters and biases for vl_nnconv don't have to be of class Param. They could be any other Layer (i.e., the output of a subnetwork), or simple Matlab arrays (and thus constant).

There is generally no restriction in what options and arguments you use, since the list of arguments is stored as-is. This property extends to other functions, and to any custom layers that you may define. A layer type (both standard and custom) is just a function handle that accepts arbitrary arguments. To execute it in backward mode, computing a derivative, AutoNN will simply pass it an extra derivative argument, which can be easily detected by the function to act accordingly.

Math functions

The Layer class overloads many math operators and native Matlab functions. Their derivatives are defined, so that it is possible to backpropagate derivatives through complex formulas.

Let's say one of your colleagues suggested that you test your network with weight normalization. Scanning the paper, you see that it consists of normalizing the filters to unit norm, and then multiplying by a learned scalar parameter g, before feeding them to the convolution (eq. 2).

Normally you'd need to worry about computing the derivatives and creating some variant of the convolution layer. However, using the overloaded math operators in AutoNN, you can simply write down the formula:

filters = Param('value', randn(5, 5, 1, 20, 'single'));
g = Param('value', 1);
filters_wn = (filters ./ sum(filters(:).^2)) * g;

These filters can then be used in a convolutional layer:

conv1_wn = vl_nnconv(images, filters_wn, biases);

During network evaluation, the derivatives will be backpropagated correctly through the math operators and into the learnable parameters.

The full list of overloads can be seen with methods('Layer'), or here. Some examples are array slicing and concatenation, element-wise algebra, reduction operations such as max or sum, and matrix algebra, including solving linear systems.

Network evaluation

To run as efficiently as possible, the computational graph must be compiled into a simple sequence of instructions. This is stored as an object of class Net.

First, we will finish the network by adding a softmax loss:

labels = Input();
loss = vl_nnloss(conv2, labels);

To help debug the compiled network, and identify our inputs, we would like to assign nice names to the layers we defined - preferably the same names as the variables we used. This can be done automatically:

Layer.workspaceNames();

You can then check that loss.name = 'loss', and so on for all the other workspace variables. Only previously unnamed layers will be set.

Alternatively, loss.sequentialNames() would fill in unnamed layers involved in the computation of loss, using intuitive names like convN for the N'th convolutional layer, and convN_filters for the corresponding filters. To ensure that all layers have names, network compilation will call this function.

Finally, we compile the network by passing the loss to the Net constructor:

net = Net(loss);

This is now ready to run! Use the eval method to process some inputs, performing backpropagation:

net.eval({'images', rand(28, 28, 'single'), 'labels', 1});

The derivative of the loss with respect to a variable (a layer's output) can then be accessed with getDer:

net.getDer('pool1')

The access methods for network variables are getValue/setValue, and for their derivatives getDer/setDer.

For serious training tasks, we should use the highly optimized GPU routines of MatConvNet and Matlab's gpuArray. We can mark the images input to be automatically transferred to the GPU, as follows:

images.gpu = true;

Note that this has to be done before compiling the network. The gpu property of Param objects is true by default. To actually enable the GPU computations on the GPU with index idx, use:

net.useGpu(idx);

Using these elements, we can compose an SGD training loop. Simple examples of such loops are included in the directory examples/minimal. For more demanding tasks, it's probably best to use the AutoNN training packages: solvers, datasets, and models. These packages contain classes that can be mixed and matched freely. Examples on how to use them can be found in the examples/cnn and examples/rnn directories. To see a list of all the available solvers, for example, type help solvers into the console. A full list of all classes and methods, and their documentation, can be accessed with help autonn.

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