Fix DDP issue where parameters share same grad_accumulator (#46755)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46755
As reported in https://github.com/pytorch/pytorch/issues/41324, there is a bug in DDP when `find_unused_parameters=True` and 2 or more parameters share the same gradient accumulator.
In the reducer, we currently keep a mapping of grad accumulator to index and populate it with map[accumulator] = index, but this overwrites indices when the accumulator is the same. To fix this, switch the mapping values to a vector of indices to hold all such indices that share the same accumulator.
ghstack-source-id: 115453567
Test Plan: Added UT
Reviewed By: pritamdamania87
Differential Revision: D24497388
fbshipit-source-id: d32dfa9c5cd0b7a8df13c7873d5d28917b766640
