Fix DDP Example for CIFAR10 by Using Epochs Only

[ImahnShekhzadeh]

Hi!

As recently discussed in #145 and #144 with @Xiaoming-Zhao (and I as had already mentioned in #116 (comment)), I/we believe that it would be best to use epochs instead of steps in the distributed mode, i.e. in the training loop of train_cifar10_ddp.py. Is there a reason not to (except consistency to the other scripts)? 🙃

I would be happy to make the changes, test them extensively and then open a PR.

Cheers,
Imahn

[atong01]

Yes I agree with this change in principle. Happy to take a PR in this direction. Thanks for all your work on this example!

[kilianFatras]

Of course. I might do it if I have time but it is rather uncertain for the end of the year.