Use torch._check instead of a test to make the model Gemma3 exportable

[xadupre]

What does this PR do?

torch.export.export fails on a tests used to raise an exception if not true. This is replaced by torch._check to avoid torch.export.export complain about it.

Fixes # (issue)

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?

[xadupre]

cc @tugsbayasgalan

[github-actions]

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

run-slow: aria, aya_vision, chameleon, cohere2_vision, emu3, fuyu, gemma3, got_ocr2, internvl, llava, mistral3, paligemma, paligemma2, vipllava