Let DDP.train() return self to stay consistent with nn.Module (#42131)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/42131
Test Plan: Imported from OSS
Reviewed By: pritamdamania87
Differential Revision: D22775311
Pulled By: mrshenli
fbshipit-source-id: ac9e6cf8b2381036a2b6064bd029dca361a81777
