Update Pixtral to use SDPA
Author: mgoin
Pull request: yes

# What does this PR do?

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Fixes large memory consumption and performance issues with unfused attention in Pixtral. The usage comes from the vLLM implementation in https://github.com/vllm-project/vllm/pull/9597


## Before submitting
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Models:

- text models: @ArthurZucker
- vision models: @amyeroberts, @qubvel
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Library:

- flax: @sanchit-gandhi
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Comments:
cc @zucchini-nlp - should we keep the eager implementation around as well rather than making SDPA the only option?