improve error message on incorrect inputs into gather for (#27439)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27439
When users call dist.gather, they have to pass in a `gather_list` to
the function on the destination worker, and this list needs to have the same
size as the number of processes in the group. When the user initializes this
list incorrectly, the current error message is not very helpful:
This changes the error message so that the incorrect gather_list size is
pointed out and the correct one is given.
ghstack-source-id: 91413442
Test Plan: Added a unit test and tested with an incorrect gather_list size.
Differential Revision: D17781370
fbshipit-source-id: b49aad1b1197daf77daa10911296664e6340e2fa
