google/mt5 optimization and fix (#15454)
### Description
<!-- Describe your changes. -->
1. enabled self-attention fusion in mt-5 decoder graph
2. fix a parity issue
https://github.com/microsoft/onnxruntime/issues/15042
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
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Co-authored-by: Ubuntu <wy@v100-2.0cdb2e52twzevn1i4fi45bylyg.jx.internal.cloudapp.net>
