TrainingPhase

			TrainingPhase() <: AbstractTrainingPhase

A regular training phase for supervised learning. It iterates over batches
in learner.data.training and updates the model parameters
using learner.optim after calculating the gradients.

Throws the following events in this order:

• EpochBegin when an epoch starts,

• StepBegin when a step starts,

• LossBegin after the forward pass but before loss calculation,

• BackwardBegin after loss calculation but before backward pass,

• BackwardEnd after the bacward pass but before the optimization
  step,

• StepEnd when a step ends; and

• EpochEnd when an epoch ends

It writes the following step state to learner.state , grouped by the
event from which on it is available.

• StepBegin :

  • xs and ys : encoded input and target (batch)

• LossBegin :

  • ŷs : model output

• BackwardBegin :

  • loss : loss

• BackwardEnd :

  • grads : calculated gradients