What does this PR do?

The previous PR #20589 incorrectly returned a list of forced decoder ids:

from transformers import WhisperProcessor

processor = WhisperProcessor.from_pretrained("openai/whisper-tiny.en")
print(processor.get_decoder_prompt_ids(task="transcribe"))

Print Output:

[50257, 50358, 50362]

The correct format is a nested list of decoder ids, where the first element of each list specifies the position of the forced token and the second the token id:

print(processor.get_decoder_prompt_ids(task="transcribe"))

Print Output:

[(1, 50257), (2, 50358), (3, 50362)]

(at position 1 we force token 50257, at 2 we force 50358, at 3 we force 50362)

The PR also implements a test, thus making sure that no such error can be made again 😅

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?

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