[Whisper] Move decoder id method to tokenizer
# What does this PR do?

Moves the method `get_decoder_prompt_ids` from the processor to the tokenizer. The primary reason for this change is that the ASR pipeline class does not load the processor object, but rather the feature extractor and tokenizer separately (see [docs](https://github.com/huggingface/transformers/blob/699e90437f984d69ad3c9b891dd2e9d0fc2cffe4/src/transformers/pipelines/automatic_speech_recognition.py#L123)). Therefore, as things currently stand, pipeline does not have access to the processor method `get_decoder_prompt_ids`. By moving it to the tokenizer, it will be able to call this method with pipeline.

Note that this is not a breaking change: we retain a method `get_decoder_prompt_ids` in the processor. This method simply calls the `get_decoder_prompt_ids` from the tokenizer:

https://github.com/huggingface/transformers/blob/ca8b332d31a1b90e18f134620e69063418add69e/src/transformers/models/whisper/processing_whisper.py#L44-L45


## 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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      [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?


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