[FileStore] Updating Docs to Reflect FileStore changes (#49557)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49557
Updating the PyTorch docs to reflect that FileStore now supported the
num_keys API. Also included a note to describe the behavior of the API.
Test Plan: build and rendered docs.
Reviewed By: jiayisuse
Differential Revision: D25619000
fbshipit-source-id: 6c660d7ceb32d1d61024df8394aff3fcd0b752c1
