[distributed] Make DDP work with python process group (#79176)
This PR enables python process group usage with DDP by doing the following:
- Surface PG::Work::getFuture() as overridable()
- Use Work::getFuture() to retrieve values from a PG.
- Add _create_work_from_future python method that creates a Work object that wraps a Future.
To test this changes we use both strategies to run DDP with a python based PG.
The reason for offering two methods is that both have short-comings.
The wrapper method is harder to troubleshoot as there's no visibility of how the future is used.
The subclass method has memory management issues as can be noticed in the test suite by having to keep Work instances alive by storing them in PG fields.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79176
Approved by: https://github.com/rohan-varma
