[Gradient Compression] Minor update of the comments on PowerSGD. (#49246)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49246
Previously the comment on matrix_approximation_rank was in PowerSGD_hook function. Now move it into PowerSGDState, because the function arg is already moved to this state as an attribute.
Original PR issue: Investigate Applying PowerSGD to Communication Hook for Gradient Compression #47202
ghstack-source-id: 118414247
Test Plan: N/A
Reviewed By: rohan-varma
Differential Revision: D25501091
fbshipit-source-id: 701e3109a9a3f2a5f9d18d5bf6d0a266518ee8ea
