Raise exceptions instead of using asserts in modeling_openai #12789

[nbertagnolli]

What does this PR do?

Replaces control flow assertions in modeling_openai.py to address issue #12789.

Contributes to #12789

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

Tagging @sgugger I saw you commenting on the issue and some other PRs for this issue : ).

[sgugger]

Thanks for your help on this!