improve error message for scatter in processGroupGloo (#27458)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27458
Same as the previous diff - improve error message by passing back the
size discrepancy.
ghstack-source-id: 91864213
Test Plan: `python test/test_c10d.py`
Differential Revision: D17785296
fbshipit-source-id: f939b8091aede768ea215f69df2c83e438c430cf
