Persist IterableDataset epoch in workers

[lhoestq]

Use shared memory for the IterableDataset epoch.

This way calling ds.set_epoch() in the main process will update the epoch in the DataLoader workers as well.
This is useful especially because the epoch is used to compute the effective_seed used for shuffling.

I used torch's shared memory in case users want to send dataset copies without shared memory using pickle. I also find it easier to use than using multiprocessing.shared_memory than requires unlinking only in the main process, or mp.Value that is not picklable.

close #6673

cc @rwightman

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.