[Model Averaging] Revert #63895 (#64903)

[Model Averaging] Revert #63895 (#64903)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64903

Fix the accuracy regression caused by https://github.com/pytorch/pytorch/pull/63895.

Test Plan:
buck test mode/dev-nosan //caffe2/test/distributed:distributed_nccl_spawn -- test_periodic_model_averager
buck test mode/dev-nosan //caffe2/test/distributed:distributed_nccl_spawn -- test_post_localSGD_optimizer_parity

Reviewed By: rohan-varma

Differential Revision: D30894688

fbshipit-source-id: fe00b8b23b860d9f806f87c1b6caba1d0b807485