Traces

			Traces(preprocess[, phase])

Record a trace during phase by apply each pre-processing function in preprocess to the Learner to produce a trace value. The trace is recorded at the end of each learning step.

See LogTraces for logging of the trace values.

			
			cb = Traces((loss2 = learner -> learner.step.loss^2,
             avg_gnorm = learner -> mean(map((_, g) -> norm(g), pairs(learner.step.grads))))
            TrainingPhase)