[torch.futures] Add note about error handling for non-chained futures. (#53212)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53212
Ran into a strange issue with error handling in future callbacks, more
details in https://github.com/pytorch/pytorch/issues/52132, but essentially,
after a callback throws all additional processing stops, and other futures can
never be completed, resulting in a hang. Add a note to warn about this.
ghstack-source-id: 123122890
Test Plan: CI
Reviewed By: SciPioneer
Differential Revision: D26793310
fbshipit-source-id: b1ae73a81163d7b37ba07b0685e8de4228f01da6
