[FSDP] Rename streams (#86833)
This time around, I decided to rename the "all_gather" stream to the "unshard" stream to emphasize that it includes both the actual all-gather op but also the corresponding memory allocations (and also now the unflattening as well). (A similar reasoning applies for the "pre-all-gather" stream becoming the "pre-unshard" stream.)
This PR is definitely safe.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86833
Approved by: https://github.com/rohan-varma
