Fix GradBucket Typing (#52943)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/52943
Test Plan: Imported from OSS
Reviewed By: rohan-varma
Differential Revision: D26699759
Pulled By: mrshenli
fbshipit-source-id: 712165a29d114da761ef4f161096ca46a958df03
