Fix layernorm ad formula on weight and bias (#18233)
Summary:
Fix the layernorm formula when weight and bias passed in.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18233
Differential Revision: D14760375
Pulled By: wanchaol
fbshipit-source-id: d6bd3b137bc04c391aa5c24d021d1f811ba2a877
