[Model Averaging] Create a base class for model averaging (#62111)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62111
This base class will be passed to the post-localSGD optimizer in the next PR. This way, the same post-localSGD optimizer can choose different model averaging algorithms.
Proposal: https://github.com/pytorch/pytorch/issues/59699
ghstack-source-id: 134489187
Test Plan: buck test mode/dev-nosan caffe2/test/distributed:distributed_nccl_fork -- test_periodic_model_averager
Reviewed By: rohan-varma
Differential Revision: D29884954
fbshipit-source-id: 1dc5e35c58895902991567f633afd621c7108938
