Move LayerNorm op schema to c10 (#15199)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15199
In order to call it from PyTorch, this op schema can't live in caffe2 but must be included from PyTorch.
Moving it to c10. This is not where it should be in the end (that's why there is a large TODO here),
but an intermediate hack to enable this use case and proof-of-concept.
Reviewed By: ezyang
Differential Revision: D13462124
fbshipit-source-id: 1e187b9def8ef049c91e6de947ea4a85758d711b
