ValidationPhase
	
struct defined in module 
	FluxTraining.Phases
			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
There is
			1
			method for FluxTraining.Phases.ValidationPhase:
		
The following pages link back here:
training/onecycle.jl , FluxTraining.jl , callbacks/metrics.jl , callbacks/phases.jl , callbacks/trace.jl , training.jl