[dist_optim] introduce distributed functional optimizer (#45221)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45221
This PR introduces a distributed functional optimizer, so that
distributed optimizer can reuse the functional optimizer APIs and
maintain their own states. This could enable the torchscript compatible
functional optimizer when using distributed optimizer, helps getting rid
of GIL and improve overall performance of training, especially distributed
model parallel training
Test Plan: Imported from OSS
Reviewed By: ailzhang
Differential Revision: D23935256
Pulled By: wanchaol
fbshipit-source-id: 59b6d77ff4693ab24a6e1cbb6740bcf614cc624a
