Add a test to ensure DDP join works with RPC (#44439)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44439
Adds a test to ddp_under_dist_autograd_test to enusre that that uneven
inputs join() API works properly when DDP + RPC is combined. We test that when
running in outside DDP mode (DDP applied to whole hybrid module) we can
correctly process uneven inputs across different trainers.
ghstack-source-id: 112156980
Test Plan: CI
Reviewed By: albanD
Differential Revision: D23612409
fbshipit-source-id: f1e328c096822042daaba263aa8747a9c7e89de7
