[DDP] fix param to name mapping (#57771)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57771
This mapping didn't work properly when certain parameters didn't
require grad. Fixed that and added a test.
ghstack-source-id: 128527537
Test Plan: CI
Reviewed By: SciPioneer
Differential Revision: D28265636
fbshipit-source-id: 7b342ce012b2b7e33058b4c619ffb98992ed05b7
