Fix flaky test vision encoder-decoder generate

[zucchini-nlp]

What does this PR do?

Fixes #28841. I could not exactly reproduce the failing test, but the issue seems to be the EOS stopping generation earlier than it reaches max_length.

Before adding the fix I checked that EOS was indeed indicated in the generation_config as not None.

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@gante

[HuggingFaceDocBuilderDev]

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[gante]

Makes sense, thank you for looking into it!

[amyeroberts]

Thanks for looking into this and fixing!