[Model Averaging] Move step out of model averaging API (#60632)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60632
Address the comment https://github.com/pytorch/pytorch/pull/60320#discussion_r654845062
ghstack-source-id: 132340278
Test Plan: buck test mode/dev-nosan caffe2/test/distributed:distributed_nccl_fork -- test_periodic_model_averager
Reviewed By: rohan-varma
Differential Revision: D29355609
fbshipit-source-id: 50a6f13ed70b5a5b5b92ead2f3d7082c11277af5
