Do rowwisemoments computation in `float` for `half` `LayerNorm` (#66920)
Summary:
https://github.com/pytorch/pytorch/issues/66707
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66920
Reviewed By: mrshenli
Differential Revision: D31850612
Pulled By: ngimel
fbshipit-source-id: a95a33567285dcf9ee28d33f503cead3268960f9
