Replace tokenizer to processing_class in Seq2SeqTrainer

[zzaebok]

What does this PR do?

This PR replace tokenizer usage in Seq2SeqTrainer to processing_class to prevent from showing warning "Trainer.tokenizer is now deprecated. You should use Trainer.processing_class instead."

Fixes # (issue)

#35446

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@LysandreJik

[LysandreJik]

Awesome, thank you! cc @qubvel as I believe you originally implemented self.processing_class

[HuggingFaceDocBuilderDev]

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