Single-node ImageNet DDP

[briankosw]

An example of DDP for ImageNet using multirun, as discussed with @omry in the Hydra repo.

[briankosw]

@omry, let me know if you think another issue for torch.distributed + multirun will be useful here too.

[omry]

@briankosw, since you already have the other example read you can just send a PR directly.

[romesco]

@briankosw If you have a draft PR, I'm happy to check it out! =] Doesn't need to be finished.

[briankosw]

@omry @romesco sorry for the delay, I've transcribed the ImageNet DDP code and added Hydra configurations to it (a lot of it was inspired by the helpful MNIST example).