Past Keys Output now working with output router logits

[csking101]

What does this PR do?

This handles the case where output_router_logits are True, and past key values are being used for inference, in the Mistral model.

Fixes #30731

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• 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?

Who can review?

@ArthurZucker

[ArthurZucker]

Hey! Sorry but this seems to be a little bit too complicated for a fix that should be simpler! 🤗

[csking101]

Hi @ArthurZucker , could you give me a pointer as to how to go about it, as I think I confused myself with the approach. The main reason why I did this was, because I was unsure about what to do with the missing attention for the previous outputs, when running the model in inference mode.