Fix promotion bug in MultiHeadDotProductAttention:

[giovannic]

If x64 is enabled, autoregressive decoding would lead to dynamic slicing errors

Changes:

• Add explicit types for dynamic_slice arguments
• Add test for autoregressive decoding with x64 enabled

Please refer to the test case to recreate.

What does this PR do?

Fixes # (issue)

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other
  checks if that's the case).
• This change is discussed in a Github issue/
  discussion (please add a
  link).
• The documentation and docstrings adhere to the
  documentation guidelines.
• This change includes necessary high-coverage tests.
  (No quality testing = no merge!)

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[cgarciae]

Thanks @giovannic for doing this!

(@chiamp we should port this to NNX as well)

[giovannic]

Thanks for the review! Should be resolved.

[chiamp]

Thanks @giovannic for doing this!

(@chiamp we should port this to NNX as well)

Sounds good, I added it in #3576