[Model Averaging] Simplify PostLocalSGD Optimizer API (#64885)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64885
1) The constructor accepts a local optimizer instance instead of the inputs of local optimizer constructor and the class type.
2) The parameters are read from local optimizer's `param_groups` instead of a separate input.
Proposal: https://github.com/pytorch/pytorch/issues/59699
ghstack-source-id: 137865867
Test Plan: buck test mode/dev-nosan //caffe2/test/distributed:distributed_nccl_spawn -- test_post_localSGD_optimizer_parity
Reviewed By: rohan-varma
Differential Revision: D30888794
fbshipit-source-id: 21261b480f6bbb9b2333426020e3f350da3f73c2
