ValidationPhase

			ValidationPhase()

A regular validation phase. It iterates over batches
in learner.data.validation and performs a forward pass.

Throws the following events: EpochBegin , StepBegin ,
LossBegin , StepEnd , EpochEnd .

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,

• 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

• StepEnd :

  • loss : loss