[c10d] Change pg/process_group to self (#80978)
Summary:
Change the naming of pg/process_group to self in ProcessGroup's py class.
Essentially pg and process_group all mean self. Therefore, it's confusing
to use a different naming convention.
Test Plan:
Covered by existing unit tests.
Fixed #80246.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80978
Approved by: https://github.com/mrshenli
