Make DistributedDataParallel usable with CPU models (#20236)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20236
Use the new version of broadcast_coalesced that deals with both CPU
and CUDA models. Add tests that evaluate correctness of
DistributedDataParallel for CPU models.
Closes #17757.
Reviewed By: mrshenli
Differential Revision: D15245428
fbshipit-source-id: d2fa09f68593b3cd1b72efeb13f5af23ebd5c80a
