Fix audio-related config naming for Gemma3n 
Author: ywang96
Pull request: yes

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While working on adding multimodal support for Gemma3n on vLLM, I noticed `audio_soft_tokens_per_image` should really be renamed to `audio_soft_tokens_per_audio`. This PR updates the variable naming but we'll also need to update this field in the model repository.


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Comments:
I've made corresponding model config PRs in all model repositories

https://huggingface.co/google/gemma-3n-E4B-it/discussions/12
https://huggingface.co/google/gemma-3n-E4B/discussions/3
https://huggingface.co/google/gemma-3n-E2B-it/discussions/8
https://huggingface.co/google/gemma-3n-E2B/discussions/3

Maybe @zucchini-nlp you can take a look and let me know what you think? Thanks!