[Whisper] Move decoder id method to tokenizer
Author: sanchit-gandhi
Pull request: yes

# 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
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- [ ] Did you write any new necessary tests?


## Who can review?

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members/contributors who may be interested in your PR.

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