[Whisper Tokenizer] Make more user-friendly
# What does this PR do?

Fixes #19864.

In summary, the Whisper tokenizer is modified to prepend several tokens to the start-of-sequence:
- BOS token id (`<|startoftranscript|>`) -> consistent with other sequence-to-sequence models such as _BART_.
- Language token id (e.g. `<|es|>` for Spanish) -> set only when the tokenizer is instantiated with argument `language=X`. Otherwise omitted.
- Task token id (e.g. `<|translate|>` for speech translation) -> set only when the tokenizer is instantiated with argument `task=Y`. Otherwise omitted.
- No time stamps id (`<|notimestamps|>`)  ->set only when the tokenizer is instantiated with argument  `predict_timestamps=False`. For `predict_timestamps=True`, it is omitted.

In addition, it is modified to always append the end-of-sequence token to the end of the label sequence (`<|endoftext|>`).

The updated tokenizer behaves as follows:
```python
from transformers import WhisperTokenizer
tokenizer = WhisperTokenizer.from_pretrained("openai/whisper-tiny", language="english", task="transcribe", predict_timestamps=False)

input_ids = tokenizer("hey").input_ids

text_with_special = tokenizer.decode(input_ids, skip_special_tokens=False)
text = tokenizer.decode(input_ids, skip_special_tokens=True)

print("Input ids :", input_ids)
print("Text w/ special :", text_with_special)
print("Text :", text)
```
**Print Output:**
```
Input ids : [50258, 50259, 50359, 50363, 17230, 50257]
Text w/ special : <|startoftranscript|><|en|><|transcribe|><|notimestamps|>hey<|endoftext|>
Text : hey
```

The attention mask functionality of the Whisper tokenizer **is** retained (_c.f._ https://github.com/huggingface/transformers/issues/19864#issuecomment-1291799687).

## 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).
- [x] 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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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

 -->

_The documentation is not available anymore as the PR was closed or merged._<|||||>I'm not sure if Patrick currently has the bandwidth to review this, @sgugger would you be able to take a look if you've got a spare few minutes? Thanks! 🙏<|||||>Test for `set_prefix_tokens` in https://github.com/huggingface/transformers/pull/19921/commits/e98821f12ba9a899d2907ebcb7d114aff8712c0b<|||||>Cool good to merge for me