[Whisper] Fix decoder ids methods
Author: sanchit-gandhi
Pull request: yes

# What does this PR do?

The previous PR https://github.com/huggingface/transformers/pull/20589 incorrectly returned a list of forced decoder ids:
```python
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:
```python
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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      Pull Request section?
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      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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