Added serialization to postlocal_SGD. (#80435)
Fixes #75666
Current PR adds the functionality for `PostLocalSGD` communication hook and tests that communication hook can be properly saved and restored. Similar to https://github.com/pytorch/pytorch/pull/79334, where serialization was added to `PowerSGD`.
``__getstate__``
Returns:
```
``Dict[str, Any]`` which will be pickled and saved.
``process_group`` and ``subgroup`` are not serializable and excluded from
a returned state.
```
``__setstate__``
```
Takes provided ``state`` and retrieves ``PostLocalSGDState``.
``process_group`` and ``subgroup`` are set to default process_group and subgroup respectively.
Default subgroup is equivalent to the subgroup on each node.
```
Small adjustment to `PowerSGD`'s warning message.
Refactored unittest, i.e. separated parity and log checks.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80435
Approved by: https://github.com/awgu
