[torch.futures] Clarify callback behavior when future is completed (#50978)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50978
Noticed that the documentation is not clear that the cbs are invoked
inline if the future is already completed. We should probably document this
behavior.
ghstack-source-id: 121021432
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D25944636
fbshipit-source-id: f4ac133d076ba9a5690fecfa56bde6d614a40191
