Add flex_attn to diffllama

[muellerzr]

What does this PR do?

Should solve failing consistency checks

Fixes # (issue)

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@ArthurZucker

[ArthurZucker]

Thanks

[ArthurZucker]

(Modeling file is not changed)

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[Cyrilvallez]

Actually I don't think DiffLlama supports flex, does it? 🤔 It's still in the old format with classes

[ArthurZucker]

mmmm yeah looks like it but the pretrained mode in modular is a bit wrong in that case