[Reducer] move comment to the right place (#58594)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58594
This comment was misplaced after some changes, move it to the right
place.
ghstack-source-id: 129498228
Test Plan: ci
Reviewed By: zhaojuanmao
Differential Revision: D28548100
fbshipit-source-id: a9163fc3b25a9d9b8b6d4bfa2a77af290108fc09
