Fix ProphetNet forward to handle tuple encoder_outputs

[Abdennacer-Badaoui]

This PR fixes a bug in ProphetNet where passing encoder_outputs as a tuple to forward() would raise:

AttributeError: 'tuple' object has no attribute 'last_hidden_state'

This happens because the forward() method assumes encoder_outputs is a BaseModelOutput when provided manually, while the docstring says it can be a tuple.

Changes:

Added a check in forward():

Convert tuple encoder_outputs to BaseModelOutput format before passing to ProphetNetSeq2SeqModelOutput.

if isinstance(encoder_outputs, tuple):
    encoder_outputs = BaseModelOutput(
        last_hidden_state=encoder_outputs[0],
        hidden_states=encoder_outputs[1] if len(encoder_outputs) > 1 else None,
        attentions=encoder_outputs[2] if len(encoder_outputs) > 2 else None,
    )

[HuggingFaceDocBuilderDev]

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[github-actions]

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

run-slow: prophetnet

[Rocketknight1]

cc @ArthurZucker for text models

[ArthurZucker]

I think we'd rather update the docstring as it should no longer be a tuple!