Add IterableDataset.shard()
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7252). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update. Is there some way to get this to work for pytorch dataloader workers?

eg. start with a single sharded IterableDataset.from_generator(), then reshard before calling map() to do expensive processing over multiple workers
Will be useful to distribute a dataset across workers (other than pytorch) like spark

I also renamed `.n_shards` -> `.num_shards` for consistency and kept the old name for backward compatibility. And a few changes in internal functions for consistency as well (rank, world_size -> num_shards, index)

Breaking change: the new default for `contiguous` in `Dataset.shard()` is `True`, but imo not a big deal since I couldn't find any usage of `contiguous=False` internally (we always do contiguous=True for map-style datasets since its more optimized) or in the wild