Fix confusing warning in EncoderDecoderModel when creating decoder_input_ids from labels

[st81]

What does this PR do?

Fixes #40168

As mentioned in the issue, it's confusing that the warning "The decoder_input_ids are now created based on the labels, no need to pass them yourself anymore" is shown even when the user doesn't pass decoder_input_ids. Fixed to show this warning only if decoder_input_ids is actually passed.

Note that the original warning message contains two parts: one about the decoder_input_ids issue, and another about the v4.12.0 change. I kept the latter warning message so it's still always shown, because it's a slight breaking change worth notifying users about (as discussed in here).

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?

• @NielsRogge (since you refactored the EncoderDecoderModel to create decoder_input_ids from labels in [Design proposal] Fix EncoderDecoderModel classes to be more like BART and T5 #14139)

[github-actions]

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

run-slow: encoder_decoder

[NielsRogge]

cc @ArthurZucker is this warning still necessary given v5 is on the roadmap?

[st81]

@ArthurZucker cc @NielsRogge

This PR has been open for about a week now. Would you mind taking a look when you get a chance? Thanks!