Make Idefics exportable, assume no padding

[jackzhxng]

What does this PR do?

Makes Idefics 3 exportable by assuming batch size of 1, unrolling the loop to a single iteration.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case. Add SmolVLM optimum-executorch#163 (comment)
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@zucchini-nlp

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: idefics3