In assisted decoding, pass model_kwargs to model's forward call
# What does this PR do?

Previously, assisted decoding would ignore any additional kwargs that it doesn't explicitly handle. This was inconsistent with other generation methods, which pass the model_kwargs through prepare_inputs_for_generation and forward the returned dict to the model's forward call.

The prepare_inputs_for_generation method can not be used directly in this case, as many implementations assume they should only keep the last input ID if a past_key_values is passed. Same goes for attention_mask etc.

The prepare_inputs_for_assisted_generation method modifies the outputs from prepare_inputs_for_generation so that they are suitable for assisted generation. This should work for most models, but if necessary a model can override this method to implement custom logic.

Fixes #25020


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

@gante 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25135). All of your documentation changes will be reflected on that endpoint.<|||||>@gante This is ready to review now. Thanks in advance.