Refactor DDP uneven inputs control flags (#47394)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47394
This is a preliminary refactor for the next diff that will add an
additional flag to control whether we throw a StopIteration or not. We
basically move the flags for ddp uneven inputs to a simple class.
ghstack-source-id: 116428177
Test Plan: CI
Reviewed By: pritamdamania87
Differential Revision: D24739509
fbshipit-source-id: 96bf41bd1c02dd27e68f6f37d08e22f33129b319
