layer_norm_fix (#15844)
### Description
Fix bugs of Layernorm Fusion.
More checks on ReduceMean axes
separate out layernorm transform_test
### Motivation and Context
Our layernorm fusion pattern works only for axis=-1 currently.
- For training senario: The pattern produced error results directly as
they didn't handle "axes" and only assumed it's the default vaue.
- For Inference:
~~We lost some oppotunities to fuse layernrom. ~~
ReduceMean has default axes 0 which means reduce on all dimensions
