Fixed the flaky test_rref_context_debug_info (#32749)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32749
The test was flaky since the message from owner RRef confirming fork would arrive after the test checked whether the pending User RRefs map was empty - leading to an assertion error. This diff creates a utility function that should be used by any test to wait for this message to complete processing before doing any assertions related to the pending User RRefs map.
GitHub Issue: https://github.com/pytorch/pytorch/issues/30988
Test Plan: Stress tested `test_rref_context_debug_info` 200 times.
Differential Revision: D19612289
fbshipit-source-id: 57a7c19b1cf792b94c263d3efbbbb6da60c07d07
