Optimize channel_stats_op (#16243)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16243
Optimize channel_stats_op and add NHWC impl
Reviewed By: takatosp1
Differential Revision: D13775515
fbshipit-source-id: decb889e646f5316d4afefdf9f9b6bc6343613cd
