[Whisper] Fix timestamp processor

# What does this PR do?
Mostly adds condition when looking in the past and in the futur based on timing information. 
Fixes the tests
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_21187). All of your documentation changes will be reflected on that endpoint.<|||||>Tested with a concatenated librispeech (clean, test, 5.4 hours), took 393.1348168849945 seconds, and with a WER 0.030776774096215136. So in that case not really sure why we are performing better.
Openai took 2661.047640323639  and had a wer of 0.2589624153733004. Which is pretty interesting (the model is `large`) so a `x6.7`speed

<|||||>Will open a PR for the other fix 