[ONNX] Fix layer_norm return type (#85979)
When aten fallback is true, `_layer_norm_returns_normalized_input_mean_rstd` can return a single value.
- Removed `_layer_norm_returns_normalized_input_mean_rstd` and have layer_norm call native_layer_norm.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85979
Approved by: https://github.com/BowenBao
