[Gradient Compression] Add cuda.syncrhonize back to batched powerSGD (#54838)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54838
Realize that an explicit sync is somehow still needed for batched PowerSGD hook. I find that a job failure can be fixed by this change.
The sync was once removed by #54482.
Test Plan:
f260900882
f260899693
Reviewed By: rohan-varma
Differential Revision: D27384738
fbshipit-source-id: 3efd738b9fd375e2ceb36ed3a6bf99cd8ce8ff95
