[ROCm][AMD] Enable ROCm Flash Attention Backend for Encoder-Decoder Models

[vllmellm]

Description

This PR enables ROCm Flash Attention for ENCODER-DECODER models.

Testing

Locally ran tests:

• vllm/tests/models/encoder_decoder/audio_language/test_whisper.py [Passed]
• vllm/tests/encoder_decoder/test_e2e_correctness.py [Passed]
• vllm/tests/models/encoder_decoder/vision_language/test_florence2.py [Passed]

[github-actions]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @vllmellm.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[hmellor]

Closing as this was superseded by #35334 which enables encoder-decoder models on both ROCm and AITER unified attention backends. Thanks for the contribution!