fix: handling of default attrs in SimplifiedLayerNormalization + LayerNormalization🐛 (#2396)

`SkipLayerNormFusion` does currently not fuse ops, if stash_type is at
default (=1) or epsilon is at default (=1e-5) for
[`LayerNormalization`](https://onnx.ai/onnx/operators/onnx__LayerNormalization.html#)
and `SimplifiedLayerNormalization`

This pr:
- fixes handling default attrs in `LayerNormalization`,
`SimplifiedLayerNormalization`
- adds BART encoder as new test model. I added this model as some of the
stash types are at default. The model is versatile and can also be used
to test other fusions e.g., `EmbedLayerNormalization`.
- allows for commuted inputs.

Closes #2378.

@shubhambhokare1 @justinchuby Could you please review? Any feedback is
greatly appreciated.

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Co-authored-by: Justin Chu <[email protected]>

Author: KarelZe