Generate: validate `model_kwargs` on FLAX (and catch typos in generate arguments)
# What does this PR do?

FLAX version of https://github.com/huggingface/transformers/pull/18261

Adds model_kwargs validation to FLAX generate, which also catches typos in the arguments. See the PR above for more details and an example of the error message users will see.
_The documentation is not available anymore as the PR was closed or merged._