[DDP] Fix wrong call to dist.get_rank() (#53793)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53793
This call should pass in the process group so it works appropriately
for subgroups instead of whole world being passed into DDP.
Aside: This wasn't caught by tests since we don't have good testing around
passing subgroups into DDP, I believe nearly all tests use the entire world.
Should we add better testing for subgroups which may potentially bring up more
subtle bugs?
ghstack-source-id: 123640712
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D26972367
fbshipit-source-id: 8330bd51e2ad66841e4c12e96b67d3e78581ec74
