Enable typechecking for `torch.futures` (#41675)
Summary:
Add typing declarations for torch._C.Future and torch._C._collect_all
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41675
Reviewed By: izdeby
Differential Revision: D22627539
Pulled By: malfet
fbshipit-source-id: 29b87685d65dd24ee2094bae8a84a0fe3787e7f8
