Add compute_loss_func to Seq2SeqTrainer

[d223302]

What does this PR do?

Add compute_loss_func to Seq2SeqTrainer to support the compute_loss_func in Trainer.

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?

Who can review?

@muellerzr and @SunMarc

[muellerzr]

Thanks!

cc @ArthurZucker for final

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[SunMarc]

LGTM !