[Gemma3n] Fix audio batching

[NickLucche]

Fix #24006 by enabling proper batched audio_tower inference.
This is done by simply padding the sequences to the max seq len.

Thanks to @pratapyash for reporting the bug!

Test with

# vllm serve google/gemma-3n-E2B-it 
(vllm) ➜  vllm git:(gemma3n-fix-batch) ✗ python examples/online_serving/openai_chat_completion_client_for_multimodal.py -c multi-audio

Chat completion output from input audio: No, they are not the same. The first audio is of a sports broadcast announcing a baseball game. The second audio appears to be a recitation of an ancient poem, possibly related to mythology or religion, in Italian.

[NickLucche]

cc @DarkLight1337

[DarkLight1337]

Do we still need input_features in that case?

[NickLucche]

I definitely want to review that once I enable that processor test that required a hf transformer bump.
For now there's no big overhead at runtime 'cause the unpadded it's just a view.

[gemini-code-assist]

Code Review

This pull request fixes an issue with batched audio inference for Gemma3n models by padding audio sequences. The core logic involves introducing a padded version of input_features for batched processing by the audio tower, while keeping an unpadded version for caching. The changes are generally good, but I've identified a critical issue with a .squeeze(1) call that will likely cause a crash, and a high-severity issue with an incorrect type hint.

[gemini-code-assist]

The use of .squeeze(1) here is likely incorrect and will cause a runtime error. input_features_padded is expected to have a shape of (batch_size, seq_length, num_features). Calling .squeeze(1) will only succeed if seq_length is 1, which is not generally the case for audio features. This seems to be a pre-existing issue, but since this line is modified, it's important to address it. The .squeeze(1) should probably be removed.

Suggested change

	input_features = audio_input["input_features_padded"].squeeze(1)
	input_features = audio_input["input_features_padded"]

[pratapyash]

Thanks for the fix @NickLucche !

[DarkLight1337]

LGTM as long as tests still pass

[NickLucche]

@DarkLight1337 looks green