[ATen] Use expect_contiguous in layer_norm (#58067)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58067
- Use expect_contiguous in layer_norm to avoid unnecessary refcount bumps when the tensors are contiguous
- Clean up some leftovers from the hacky wrappers removal cleanup: use c10::MaybeOwned<Tensor> for bias tensors
- Skip dispatcher for at::empty in the layer_norm impl in Static Runtime
Test Plan: CI
Reviewed By: swolchok
Differential Revision: D28214298
fbshipit-source-id: 73150fa62d5c18f41a2264f8e56bbe5e377ad045
