Fix attention fusion in conformer encoder (#23711)
### Description
This PR updates the attention fusion for conformer-encoder models. It is
a follow-up to [this
PR](https://github.com/microsoft/onnxruntime/pull/23528).
### Motivation and Context
Subsequent modeling code updates have changed (and will continue to
change) the graph fusions. However, the three ending attention mask
nodes (`Cast --> Unsqueeze --> Equal`) will remain. Thus, the attention
fusion should work regardless of any future modeling code changes when
handling the attention mask.
