Fix the doc of PostLocalSGDState (#72792)
Summary:
The first arg of `PostLocalSGDState` ctor, `process_group`, cannot be empty. Here to simplify the usage, does not even create a subgroup explicitly.
See the example in unit test: https://github.com/pytorch/pytorch/blob/4feef6c97092cfde7d57a97d8390a79551e92369/torch/testing/_internal/distributed/distributed_test.py#L4260
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72792
Reviewed By: samdow
Differential Revision: D34213221
Pulled By: rohan-varma
fbshipit-source-id: 078343f3ee138e175bf835897f190032eb970662
(cherry picked from commit bf90af704fb371eef799a951007cc5d41dbe07a1)
