[FSDP][Easy] Rename streams; add back stream sharing test (#104966)
Purely out of preference, this PR renames the streams to `_unshard_stream` instead of `_streams_unshard` etc. since the former reads more naturally. The PR also removes some duplicated comments and adds back a unit test that streams are shared.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/104966
Approved by: https://github.com/rohan-varma
