[Whisper] Fix word-level timestamps error in DTW - `ValueError: too many values to unpack (expected 2)`
Author: guy1992l
Pull request: yes

# What does this PR do?
When trying to use the ASR with a whisper model, `return_timestamps="word"` and `batch_size>1`, there was an error in the method `_dynamic_time_warping`:

```
File "/transformers/models/whisper/modeling_whisper.py", line 258, in _dynamic_time_warping
    output_length, input_length = matrix.shape
ValueError: too many values to unpack (expected 2)
```

This PR resolves it. It is a small (or better suited for whisper - tiny) fix for checking the `num_frames` for its type (the `np.ndarray` type was being ignored) in order to enter the correct condition before calling the method `_dynamic_time_warping`.

If the `num_frames` shouldn't be a `np.ndarray` in the first place, then this PR should be closed and ignored. Otherwise, it is working as expected.

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I think @ylacombe can review as it is a continuation to this great fix (https://github.com/huggingface/transformers/pull/28114)
Otherwise, then by the suggestions in the PR: @sanchit-gandhi 

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I hope it is ok,
Thanks!

Comments:
Just tested this, can confirm it fixes the issue 👍 

Hey @guy1992l, thanks for opening the PR, this is indeed something that slipped through my net with my PR!

#28288 is offering the same fix +  support with MPS backend so we'll probably go with that PR ! Thanks again for your work on this 

No problem, then I'm closing this PR. Thanks!