WIP In assisted decoding, pass model_kwargs to model's forward call (fix prepare_input_for_generation in all models)
# 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 needs to be amended in all models, as previously it only kept the last input ID when a past_key_values was passed.

Fixes #25020


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      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 

@sinking-point the PR has "WIP" in the title -- is it still under development, or is it ready to review?<|||||>Not ready yet. Still have to fix more models and see what's breaking the other test. I've deprioritised this somewhat as it's quite time consuming, but I'll keep chipping away at it whenever I can.

If you need this done quickly, you're welcome to help - lmk and I'll add you as a collaborator on my branch.<|||||>Not urgent -- simply double-checking whether it was in need of a review or not :)