Word-level timestamps broken for short-form audio
Author: kamilakesbi
Pull request: yes
Labels: name: Audio

# What does this PR do?

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This PR aims at fixing issue #30224: word-level timestamps is currently broken in Whisper large/large-v2/large-v3/distil-large-v3. The problem comes from the fact that `num_frames` isn't passed to the generate method when stride is None. 

I suggest simply adding a new extra argument to the output of the `preprocess` method, called `segment_size`, which will store the input length in the case of short form audios. It will be passed to the `_forward` method and used when stride is None to compute `generate_kwargs["num_frames"] = segment_size // self.feature_extractor.hop_length`. 

This seems to fix the problem. Is there any particular test that should be run regarding this issue?

## Who can review?

@sanchit-gandhi 

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_30325). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Hi @sanchit-gandhi, 

I've added slow tests that pass. With this PR we should be  able to compute word-level timestamps for short form audio using the `automatic-speech-recognition` pipeline. 

I've noticed that word-level timestamps cannot be computed using `WhisperForConditionalGeneration` instead of the pipeline. I'll open an issue so we can fix this in another PR.

Hey @amyeroberts! would appreciate a final review here when you have time. 
It should be pretty quick to review ;) 

Hi @amyeroberts, I don't! Could you merge it ? 
Thanks!