Add a warning log when there is high skew of uneven inputs in DDP training (#45238)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45238
Adds a warning when there is much higher than expected amount of
discrepancy of inputs across different processes when running with uneven
inputs. This is because a skew in the thousands can reduce performance a
nontrivial amount as shown in benchmarks, and it was proposed to add this
warning as a result. Tested by running the tests so the threshold is hit and
observing the output.
ghstack-source-id: 112773552
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D23719270
fbshipit-source-id: 306264f62c1de65e733696a912bdb6e9376d5622
