[Bugfix]: Clean up chunked prefill logging when using whisper

[simondanielsson]

Purpose

Closes #25071.

Test Plan

1. When using whisper:

vllm serve openai/whisper-large-v3

logs should no longer mention "Chunked prefill is enabled with ...":

(APIServer pid=3140911) INFO 09-17 12:37:08 [scheduler.py:222] Chunked prefill is enabled with max_num_batched_tokens=8192.
(APIServer pid=3140911) INFO 09-17 12:37:10 [__init__.py:2790] Encoder-decoder models do not support chunked prefill nor prefix caching; disabling both.

Expecting simply

(APIServer pid=3140911) INFO 09-17 12:37:10 [__init__.py:2790] Encoder-decoder models do not support chunked prefill nor prefix caching; disabling both.

2. Should lead to no changes to the SchedulerConfig nor VllmConfig. Verify with new tests.

Test Result

1. Command:

• Tested on GPU: L4.
• Output from "test" command:

(vllm) danielssonsimon@XXXXXX:~/code/vllm$ vllm serve openai/whisper-large-v3
INFO 09-17 18:43:30 [__init__.py:216] Automatically detected platform cuda.
(APIServer pid=49917) INFO 09-17 18:43:33 [api_server.py:1813] vLLM API server version 0.10.2rc3.dev169+ge3db5ebb6.d20250917
(APIServer pid=49917) INFO 09-17 18:43:33 [utils.py:328] non-default args: {'model_tag': 'openai/whisper-large-v3', 'model': 'openai/whisper-large-v3'}
(APIServer pid=49917) INFO 09-17 18:43:42 [__init__.py:707] Resolved architecture: WhisperForConditionalGeneration
(APIServer pid=49917) `torch_dtype` is deprecated! Use `dtype` instead!
(APIServer pid=49917) INFO 09-17 18:43:42 [__init__.py:1762] Using max model len 448
(APIServer pid=49917) INFO 09-17 18:43:43 [scheduler.py:197] Encoder-decoder models do not support chunked prefill nor prefix caching; disabling both.
Fetching 1 files: 100%|█████████████████████████████████████████████████████████████████████████████████████| 1/1 [00:00<00:00, 11915.64it/s]

2. New tests pass locally.

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @simondanielsson.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[hmellor]

If this already exists in ModelConfig, why duplicate it here?

[simondanielsson]

True, we likely don't want to store it here as well.

Would an InitVar be sufficient here?

[hmellor]

The InitVar solution works.

However, in other cases like this (where two sibling configs interact) I've tended to perform those interactions in the parent's __post_init__, VllmConfig in this case. Would that work in this case?

[russellb]

That's where I had it before this change, but we end up with a confusing log message about features being enabled coming from the SchedulerConfig's post_init before VllmConfig's post_init fixed it and disabled them.

[simondanielsson]

Another option would be to perform the Chunked prefill is enabled... log in the VllmConfig, but not sure it makes sense to put it there

[hmellor]

Ah I see, thankn you for explaining. Let's stick with the initvar

[simondanielsson]

@russelb conflicts fixed now - should be good to go after CI. Thanks!