docs: add functional group_norm (#54673)
Summary:
fixes https://github.com/pytorch/pytorch/issues/34209
https://11813548-65600975-gh.circle-artifacts.com/0/docs/nn.functional.html#normalization-functions
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54673
Reviewed By: ailzhang
Differential Revision: D27328211
Pulled By: zou3519
fbshipit-source-id: 75c49849377047502962157239857ed99afe6d1e
