Fix gemma3n feature extractor's incorrect squeeze
Author: Isotr0py
Pull request: yes

# What does this PR do?

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Fixes #39911
- Only squeeze batch_size dimension for `mel_spectrogram` in gemma3n processor

TODO:
- [ ] Add test


## Before submitting
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cc @NickLucche Can you check if this fix works on your gemma3n PR? Thanks!

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_39919). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Yep that mask error looks gone now, thanks for the fix @Isotr0py !

cc @eustlb @zucchini-nlp - this makes sense to me if you're happy with it!

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

run-slow: gemma3n

run-slow: gemma3n

This comment contains run-slow, running the specified jobs: 

models: ['models/gemma3n']
quantizations: [] ...