feat[vLLM × v5]: Add audio support for the Transformers backend

[harshaljanjani]

Status

On hold awaiting completion of Transformers-side audio/ALM standardization (per maintainer feedback); will update once the interface stabilizes.

What does this PR do?

→ This PR adds support for v5 Transformers audio encoder models in the vLLM Transformers backend. These changes are deliberate and are blocked by this Transformers PR which adds prerequisite compatibility to the supported models for vLLM. Once that PR is merged, this PR will be marked ready for review!
→ Outlining the design choices of one PR without context from the other didn't make much sense to me, so I wrote a doc that outlines both sets of changes together and explains their deliberate nature, amongst other valuable things!
→ The v5 tracker doesn’t mention the audio backend, but it is certainly a significant gap that needs to be addressed. After this is merged, I'll open an issue tracker for the Transformers audio backend work in vLLM so the efforts can stay organized.

Please refer to the document for the reasoning behind these changes in context with the Transformers PR!
Document: v5 x vLLM Audio Backend Support Document

Performance Metrics (Env mentioned in the document)

Reference Audio Transcript:
“MISTER QUILTER IS THE APOSTLE OF THE MIDDLE CLASSES AND WE ARE GLAD TO WELCOME HIS GOSPEL”

Model	Output Text	Latency (E2E)	Throughput	Tokens
GLM-ASR-Nano-2512	"Mister Quilter is the apostle of the middle classes, and we are glad to welcome his gospel."	856.3 ms	26.9 tok/s	23
Audio-Flamingo-3-HF	"The content of the input audio is 'mister quilter is the apostle of the middle classes and we are glad to welcome his gospel'."	1779.6 ms	16.9 tok/s	30
VibeVoice-ASR-HF	[{"Start":0,"End":5.0,"Speaker":0,"Content":"Mr. Quilter is the apostle of the middle classes, and we are glad to welcome his gospel."}]	2577.9 ms	17.1 tok/s	44
Granite-Speech-3.3-2B	"Mister Quilterter is the apostle of the middle classes, and we are glad to welcome his gospel. In written format: Mister Quilterter is the apostle of the middle classes, and we are glad to welcome his gospel."	3024.9 ms	19.5 tok/s	59

Related Issues:

→ Current v5 tracker: #38379
→ #38902
→ Solved out of the box with this PR: #32823
→ Documented vLLM engine issue mentioned in the document: #17676

@vasqu (Transformers)
@DarkLight1337 @hmellor (vLLM)

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[gemini-code-assist]

Code Review

This pull request adds support for audio models to the Transformers modeling backend by unwrapping nested CausalLM structures, extending multimodal processing metadata for audio, and refactoring embedding logic to handle audio features. It also includes a comprehensive test suite for audio model processing. A critical issue was identified in the extraction of audio embeddings, where the current implementation incorrectly selects pooled outputs instead of the full feature sequence, potentially causing a mismatch with prompt placeholders.

[gemini-code-assist]

There is an inconsistency between how audio and vision embeddings are extracted from model outputs. For vision models (line 642), the first element [0] is used, which typically corresponds to the last_hidden_state (the sequence of features). However, for audio models here (line 591), the second element [1] is used. Furthermore, line 593 explicitly selects pooler_output.

In many Transformers models, the second element of the output tuple (or the pooler_output attribute) is a single vector for the entire sequence. Using a single pooled vector instead of the full sequence of features will cause a mismatch with the number of placeholder tokens in the prompt (calculated at line 283) and lead to incorrect model behavior or poor performance. Please verify if last_hidden_state (or [0]) should be used here to ensure the full sequence of features is captured, consistent with the vision path.

Suggested change

	if isinstance(audio_output, tuple):
	audio_embeddings = audio_output[1]
	elif hasattr(audio_output, "pooler_output"):
	audio_embeddings = audio_output.pooler_output
	else:
	audio_embeddings = audio_output
	if isinstance(audio_output, tuple):
	audio_embeddings = audio_output[0]
	elif hasattr(audio_output, "last_hidden_state"):
	audio_embeddings = audio_output.last_hidden_state
	else:
	audio_embeddings = audio_output

[harshaljanjani]

Flagging this: tmk granite_speech sets audio_outputs.pooler_output = projected_embeds, i.e. the projected embeddings (the full sequence), not a single vector; probably a hallucination or codebase knowledge before its training cutoff.

[eustlb]

yep you're right, it's been adopted in transformers that pooler_output holds the encoder projected hidden states, but honestly, this always looked a bit odd and misleading to me (and not aligned with how it's documented), that's likely why it's hallucinating here

[hmellor]

Thank you for this PR!

I'm aware that @eustlb is actually doing some refactoring on the Transformers side to make audio models look more like other multimodal models (which may render the changes in causal.py unnecessary.

We should wait for this standardisation to be completed and then we can update the PR on the vLLM side to hook into this more standardised interface.

[harshaljanjani]

I'm aware that @eustlb is actually doing some refactoring on the Transformers side to make audio models look more like other multimodal models (which may render the changes in causal.py unnecessary.

I would love to provide some extra bandwidth in that regard as well @eustlb!

We should wait for this standardisation to be completed and then we can update the PR on the vLLM side to hook into this more standardised interface.

Sure, will be on the lookout for pings and updates.

[RocketRider]

Tf5 support is now merged

[eustlb]

Thanks @harshaljanjani for working on this!
Opened #45534 to fix the VLM/ ALM discrepancy regarding base model class that should reduce changes required here

[harshaljanjani]

Thanks @harshaljanjani for working on this! Opened #45534 to fix the VLM/ ALM discrepancy regarding base model class that should reduce changes required here

Awesome stuff @eustlb, thanks for letting me know! Will let the review rounds play out for the linked PR and start work here once it's merged to avoid a dupl of efforts. Also if I recall correctly, an issue was brought to light a couple of months back in this PR with traces; I'd love to know if there has been any standardization in that regard since we postponed the hotfix at the time :)
Would love to coordinate efforts, in any case happy to provide some extra bandwidth where needed!

Edit: Marking this as ready for review since the Transformers PR has now been merged. Looking forward to the review rounds once ALM standardization is complete!

[claude]

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