Get rid of deprecated use_return_dict call.

[rasmi]

What does this PR do?

config.use_return_dict is deprecated (and mostly removed in #41250) , but there is one remaining call here. This raises a torch dynamo error in certain circumstances. This PR removes the last use of this deprecated method.

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Who can review?

@zucchini-nlp
@Rocketknight1

[zucchini-nlp]

Ah nice, I thought all mentions of it were replaced 😅

[github-actions]

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

run-slow: mbart

[HuggingFaceDocBuilderDev]

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