[1/n] Thread PG: fix pyre error of class ProcessGroup (#88281)
Summary: Fix the typing stub of `ProcessGroup` in "torch/distributed/__init__.py", so that it won't confuse pyre, and we can remove a lot of pyre suppression comments.
Test Plan: pyre check
Differential Revision: D40921667
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88281
Approved by: https://github.com/wanchaol
