Remove ProcessGroup::getGroupRank (#19147)

Remove ProcessGroup::getGroupRank (#19147)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19147

After #14809 was merged there is no longer a need for getGroupRank.
Every ProcessGroup object has its own rank and size fields which are
accurate for the global group as well as subgroups.

Strictly speaking removing a function in a minor version bump is a big
no-no, but I highly doubt this was ever used outside of
`torch.distributed` itself. This will result in a compile error for
folks who have subclassed the ProcessGroup class though.

If this is a concern we can delay merging until a later point in time,
but eventually this will need to be cleaned up.

Differential Revision: D14889736

fbshipit-source-id: 3846fe118b3265b50a10ab8b1c75425dad06932d