Arm backend: Add layer_norm decomposition (#6288)

Summary:
Add decomposition for layer_norm, mean_dim and var_dim.

Pull Request resolved: https://github.com/pytorch/executorch/pull/6288

Reviewed By: cccclai

Differential Revision: D64473056

Pulled By: digantdesai

fbshipit-source-id: a3d4b0fbb0ea1d56b372b617f96dcbab41f72bb7