Add distributed usage to ImageNet example's README

[Quentin-Anthony]

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Following the discussions in #995 and #862, This PR adds a small usage example into the ImageNet README.

One note, however, is that specifying --gpu will trigger the print in https://github.com/pytorch/examples/blob/main/imagenet/main.py#L95-L97, which is confusing to users. Should we either remove or clarify this print?

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[hudeven]

I think the print for --gpu is useful. To reduce the confusion, we can add some clarification to the print message. Alternative: as DDP is recommended to replace DataParallel, we should probably remove the usage of DataParallel in examples and update the print message accordingly.

[Quentin-Anthony]

I think the print for --gpu is useful. To reduce the confusion, we can add some clarification to the print message. Alternative: as DDP is recommended to replace DataParallel, we should probably remove the usage of DataParallel in examples and update the print message accordingly.

I'm confused. This PR is for updating the README. What's being printed?