ToDevice

			ToDevice(movedatafn, movemodelfn) <: Callback

Moves model and step data to a device using movedatafn for step data
and movemodelfn for the model. For example ToDevice(Flux.gpu, Flux.gpu) ,
moves them to a GPU if available. See ToGPU .

By default, only moves step.xs and step.ys , but this can be extended
to other state by implementing on(::StepBegin, ::MyCustomPhase, ::ToDevice, learner) .