Add docstrings to native_channel_shuffle (#72919)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/72919
Test Plan: Imported from OSS
Reviewed By: bdhirsh
Differential Revision: D34274717
Pulled By: VitalyFedyunin
fbshipit-source-id: fa42f91ef2335e2594b19ef65d914c711f7a94fd
(cherry picked from commit a6f6fe9112aa36d711f240f720615be09e2e13de)
