Add debug info API for distributed autograd. (#30642)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30642
Adding a couple of basic metrics for distributed autograd which would
help in determining stuckness.
ghstack-source-id: 95156189
Test Plan: waitforbuildbot
Differential Revision: D18776478
fbshipit-source-id: a0556ad6fe2b7c3cd0082ee2350c1c78cafaaec5
