Prioritize raising error message about unused parameters when rebuild_buckets fails (#45933)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45933
Occasionally users run DDP with models with unused params, in this
case we would like to surface an error message telling them to run with
find_unused_params=True. However, a recent change to rebuild_buckets logic (https://github.com/pytorch/pytorch/pull/44798) made
it so that we raise a size mismatch error when this happens, but the
information about unused parameters is likely to be more useful and likely to
be the most common case of failure. Prefer raising this error over the
subsequent size mismatch errors.
ghstack-source-id: 113914759
Test Plan: Added unittest
Reviewed By: mrshenli
Differential Revision: D24151256
fbshipit-source-id: 5d349a988b4aac7d3e0ef7b3cd84dfdcbe9db675
