add num_pending_users to debug info (#31539)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31539
Adding this metric primarily because it is needed to unblock unit
tests for https://github.com/pytorch/pytorch/pull/31381. It also may be useful
to look at this metric to see the number of pending RRef forks that currently
exist.
ghstack-source-id: 96230360
Test Plan: Modified the relevant unit test.
Differential Revision: D19204158
fbshipit-source-id: 016345e52cd02cc5f46837bffd8d589ba8575f29
