.FluxTrainingrunstep

			runstep(stepfn, learner, phase) -> state

Run stepfn inside the context of a step. Calls stepfn(handle, state) where handle(e) can be called to dispatch events and state is a PropDict which step data, gradients and losses can be written to. Return state.

Takes care of dispatching StepBegin and StepEnd events as well as handling CancelStepExceptions.