correct comments in group_norm_op (#19621)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19621
Comments for group_norm_op is not accurate (i.e., the math part), this diff will fix it.
Reviewed By: BIT-silence
Differential Revision: D15048695
fbshipit-source-id: 27d41d3ae21054257967815254134849944d56ca
