imagenet benchmark measure elapsed time function issue

[Danliran]

Hi Pytorch team,
I think the measure elaspsed time function is not right.
Both trainning and validate function show the progress if i %args.print_freq == 0 in magenet/main.py . currently, the batch_time only measure one loop. I think the batch_time should measure args.print_freq loop elaspsed time.

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     # measure elapsed time
     batch_time.update(time.time() - end)
     end = time.time()

     if i % args.print_freq == 0:
         progress.display(i)

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