Implement JIT serialization of ProcessGroup (#48544)
Summary:
This diff enables JIT serialization of `ProcessGroup`, including both base `ProcessGroup` class and derived classes like `ProcessGroupNCCL`.
If a `ProcessGroup` is created via high-level APIs like `dist_c10d.frontend().new_process_group_helper()`, they are automatically serializable. If a `ProcessGroup` is created via its derived class TorchBind APIs like `dist_c10d.ProcessGroupNCCL()`, then it has to be given a name and registered with `dist_c10d.frontend().register_process_group_name` to be uniquely identifiable and serializable.
* Fixed a minor bug in new dist_c10d frontend which fails to check whether a process group is used or not
* Fixed an issue where `test_jit_c10d.py` wasn't really run due to a configuration bug. Now tests are run as a slow test (need ci-all/* branch)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48544
Reviewed By: wanchaol
Differential Revision: D25298309
Pulled By: gmagogsfm
fbshipit-source-id: ed27ce37373c88277dc0c78704c48d4c19d46d46
