remove unused is_decoder parameter in DetrAttention
Author: JayL0321
Pull request: yes

# What does this PR do?

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Fixes #24161 


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [ ] Did you write any new necessary tests?


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

_The documentation is not available anymore as the PR was closed or merged._