Generate: force caching on the main model, in assisted generation
# What does this PR do?

Fixes #23686 

Caching is a requirement in assisted generation -- we even check for it ([here](https://github.com/huggingface/transformers/blob/8f093fb799246f7dd9104ff44728da0c53a9f67a/src/transformers/generation/utils.py#L1485)).

However, it was still possible for the main model to run without cache. This PR fixes it.
_The documentation is not available anymore as the PR was closed or merged._