feat: make audio feature extractors torch.export-able
Author: The5cheduler
Pull request: yes
Labels: name: Audio

Fix #40986

Refactors the feature extraction logic for both `GraniteSpeechFeatureExtractor` and `WhisperFeatureExtractor` into separate `nn.Module` subclasses. This encapsulates the feature extraction computation in a way that is compatible with `torch.export`.

A new method, `to_exportable_module()`, was added to both feature extractor classes to return an instance of these new modules.

The original `__call__` and `_torch_extract_fbank_features` methods were updated to use these exportable modules.

Finally, new tests were added to verify that both feature extractors can be successfully exported using `torch.export`.

# What does this PR do?

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Fixes # (issue)


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Comments:
cc @eustlb @ebezzam 

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: granite_speech, whisper