Configure assistant model's generation_config with user parameters
Author: geoffrey-young
Pull request: yes

# What does this PR do?

Fixes issue where num_assistant_tokens and other parameters intended for the assistant model are not properly passed down, leaving the assistant model with default values.

Ultimately, we want to configure the assistant model properly, so that it:

1. Is properly configured with user-provided parameters
2. Inherits defaults from the main model instead of class defaults

Tests added to illustrate the issue, and detailed comments to illustrate behaviors.

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Fixes # [40739](https://github.com/huggingface/transformers/issues/40739)


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Comments:
cc @gante

thanks for taking a look [zucchini-nlp](https://github.com/zucchini-nlp).

I wouldn't say this is a high priority.  however, I did at least want to be sure to point out the inheritance issues with the assistant_model configuration API, since the effects really aren't obvious downstream.  I only noticed the issue because I was doing a deep trace to understand how speculative decoding worked :)  generating a PR to at least offer a potential path to a fix is the least I could do.

my local dev test env is small and the larger suite is blowing up even without the changes, so it's hard for me to trace what the breaking aspects are in order to assist much further.  I do appreciate you taking a look, though - many thanks.

Yeah, agreed 100%. Thanks for pointing out to the issue, I will start cleaning up generation config little by little. Hopefully for the v5 release, it will be solved