Making dist.get_default_group private for PT1 release (#14767)

Making dist.get_default_group private for PT1 release (#14767)

Summary:
When I wrote the frontend API, it is designed on not letting users use the default_group directly on any functions.  It should really be private.

All collectives are supposed to either use group.WORLD, or anything that comes out of new_group. That was the initial design.

We need to make a TODO on removing group.WORLD one day. It exists for backward compatibility reasons and adds lots of complexity.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14767

Reviewed By: pietern

Differential Revision: D13330655

Pulled By: teng-li

fbshipit-source-id: ace107e1c3a9b3910a300b22815a9e8096fafb1c