Configure assistant model's generation_config with user parameters

[geoffrey-young]

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.

Fixes # 40739

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[Rocketknight1]

cc @gante