Module - Neural network training and inference

Training a neural network involves quite a few steps. One need to specify how to feed input training data, initialize model parameters, perform forward and backward passes through the network, update weights based on computed gradients, do model checkpoints, etc. During prediction, one ends up repeating most of these steps. All this can be quite daunting to both newcomers as well as experienced developers.

Luckily, MXNet modularizes commonly used code for training and inference in the module (mod for short) package. Module provides both high-level and intermediate-level interfaces for executing predefined networks. One can use both interfaces interchangeably. We will show the usage of both interfaces in this tutorial.

Prerequisites

To complete this tutorial, we need:

pip install jupyter requests

Preliminary

In this tutorial we will demonstrate module usage by training a Multilayer Perceptron (MLP) on the UCI letter recognition dataset.

The following code downloads the dataset and creates an 80:20 train:test split. It also initializes a training data iterator to return a batch of 32 training examples each time. A separate iterator is also created for test data.

import logging
logging.getLogger().setLevel(logging.INFO)
import mxnet as mx
import numpy as np

fname = mx.test_utils.download('http://archive.ics.uci.edu/ml/machine-learning-databases/letter-recognition/letter-recognition.data')
data = np.genfromtxt(fname, delimiter=',')[:,1:]
label = np.array([ord(l.split(',')[0])-ord('A') for l in open(fname, 'r')])

batch_size = 32
ntrain = int(data.shape[0]*0.8)
train_iter = mx.io.NDArrayIter(data[:ntrain, :], label[:ntrain], batch_size, shuffle=True)
val_iter = mx.io.NDArrayIter(data[ntrain:, :], label[ntrain:], batch_size)

Next, we define the network.

net = mx.sym.Variable('data')
net = mx.sym.FullyConnected(net, name='fc1', num_hidden=64)
net = mx.sym.Activation(net, name='relu1', act_type="relu")
net = mx.sym.FullyConnected(net, name='fc2', num_hidden=26)
net = mx.sym.SoftmaxOutput(net, name='softmax')
mx.viz.plot_network(net)

Creating a Module

Now we are ready to introduce module. The commonly used module class is Module. We can construct a module by specifying the following parameters:

  • symbol: the network definition
  • context: the device (or a list of devices) to use for execution
  • data_names : the list of input data variable names
  • label_names : the list of input label variable names

For net, we have only one data named data, and one label named softmax_label, which is automatically named for us following the name softmax we specified for the SoftmaxOutput operator.

mod = mx.mod.Module(symbol=net,
                    context=mx.cpu(),
                    data_names=['data'],
                    label_names=['softmax_label'])

Intermediate-level Interface

We have created module. Now let us see how to run training and inference using module’s intermediate-level APIs. These APIs give developers flexibility to do step-by-step computation by running forward and backward passes. It’s also useful for debugging.

To train a module, we need to perform following steps:

  • bind : Prepares environment for the computation by allocating memory.
  • init_params : Assigns and initializes parameters.
  • init_optimizer : Initializes optimizers. Defaults to sgd.
  • metric.create : Creates evaluation metric from input metric name.
  • forward : Forward computation.
  • update_metric : Evaluates and accumulates evaluation metric on outputs of the last forward computation.
  • backward : Backward computation.
  • update : Updates parameters according to the installed optimizer and the gradients computed in the previous forward-backward batch.

This can be used as follows:

# allocate memory given the input data and label shapes
mod.bind(data_shapes=train_iter.provide_data, label_shapes=train_iter.provide_label)
# initialize parameters by uniform random numbers
mod.init_params(initializer=mx.init.Uniform(scale=.1))
# use SGD with learning rate 0.1 to train
mod.init_optimizer(optimizer='sgd', optimizer_params=(('learning_rate', 0.1), ))
# use accuracy as the metric
metric = mx.metric.create('acc')
# train 5 epochs, i.e. going over the data iter one pass
for epoch in range(5):
    train_iter.reset()
    metric.reset()
    for batch in train_iter:
        mod.forward(batch, is_train=True)       # compute predictions
        mod.update_metric(metric, batch.label)  # accumulate prediction accuracy
        mod.backward()                          # compute gradients
        mod.update()                            # update parameters
    print('Epoch %d, Training %s' % (epoch, metric.get()))

To learn more about these APIs, visit Module API.

High-level Interface

Train

Module also provides high-level APIs for training, predicting and evaluating for user convenience. Instead of doing all the steps mentioned in the above section, one can simply call fit API and it internally executes the same steps.

To fit a module, call the fit function as follows:

# reset train_iter to the beginning
train_iter.reset()

# create a module
mod = mx.mod.Module(symbol=net,
                    context=mx.cpu(),
                    data_names=['data'],
                    label_names=['softmax_label'])

# fit the module
mod.fit(train_iter,
        eval_data=val_iter,
        optimizer='sgd',
        optimizer_params={'learning_rate':0.1},
        eval_metric='acc',
        num_epoch=8)

By default, fit function has eval_metric set to accuracy, optimizer to sgd and optimizer_params to (('learning_rate', 0.01),).

Predict and Evaluate

To predict with module, we can call predict(). It will collect and return all the prediction results.

y = mod.predict(val_iter)
assert y.shape == (4000, 26)

If we do not need the prediction outputs, but just need to evaluate on a test set, we can call the score() function. It runs prediction in the input validation dataset and evaluates the performance according to the given input metric.

It can be used as follows:

score = mod.score(val_iter, ['mse', 'acc'])
print("Accuracy score is %f" % (score))

Some of the other metrics which can be used are top_k_acc(top-k-accuracy), F1, RMSE, MSE, MAE, ce(CrossEntropy). To learn more about the metrics, visit Evaluation metric.

One can vary number of epochs, learning_rate, optimizer parameters to change the score and tune these parameters to get best score.

Save and Load

We can save the module parameters after each training epoch by using a checkpoint callback.

# construct a callback function to save checkpoints
model_prefix = 'mx_mlp'
checkpoint = mx.callback.do_checkpoint(model_prefix)

mod = mx.mod.Module(symbol=net)
mod.fit(train_iter, num_epoch=5, epoch_end_callback=checkpoint)

To load the saved module parameters, call the load_checkpoint function. It loads the Symbol and the associated parameters. We can then set the loaded parameters into the module.

sym, arg_params, aux_params = mx.model.load_checkpoint(model_prefix, 3)
assert sym.tojson() == net.tojson()

# assign the loaded parameters to the module
mod.set_params(arg_params, aux_params)

Or if we just want to resume training from a saved checkpoint, instead of calling set_params(), we can directly call fit(), passing the loaded parameters, so that fit() knows to start from those parameters instead of initializing randomly from scratch. We also set the begin_epoch parameter so that fit() knows we are resuming from a previously saved epoch.

mod = mx.mod.Module(symbol=sym)
mod.fit(train_iter,
        num_epoch=8,
        arg_params=arg_params,
        aux_params=aux_params,
        begin_epoch=3)