[Whisper Tokenizer] Make more user-friendly
Author: sanchit-gandhi
Pull request: yes

# 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 (`
`) -> consistent with other sequence-to-sequence models such as _BART_.
- Language token id (e.g. `
` for Spanish) -> set only when the tokenizer is instantiated with argument `language=X`. Otherwise omitted.
- Task token id (e.g. `
` for speech translation) -> set only when the tokenizer is instantiated with argument `task=Y`. Otherwise omitted.
- No time stamps id (`
`)  ->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 (`
`).

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 : 



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