FIX: Use correct argument name in MultiheadAttention forward

[BenjaminBossan]

The arguments of the forward method of MultiheadAttention are called query etc. PEFT used x. Therefore, if a caller uses keywords only, the argument is not assigned, resulting in an error.

This was initially reported here:

https://github.com/huggingface/peft/issues/761#issuecomment-2818029500

Note: Other layers' forward method (like Linear) also uses incorrect names, like x instead of input, but so far no issues were reported, so I'll leave it as is for now.

[HuggingFaceDocBuilderDev]

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