Ensure DDP + Pipe works with find_unused_parameters. (#49908)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49908
As described in https://github.com/pytorch/pytorch/issues/49891, DDP +
Pipe doesn't work with find_unused_parameters.
This PR adds a simple fix to enable this functionality. This only currently
works for Pipe within a single host and needs to be re-worked once we support
cross host Pipe.
ghstack-source-id: 119573413
Test Plan:
1) unit tests added.
2) waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D25719922
fbshipit-source-id: 948bcc758d96f6b3c591182f1ec631830db1b15c
