[Misc] Enable async scheduling by default with spec decoding

[njhill]

Now that all of the gaps have been addressed in async scheduling + spec decoding support, we can enable it by default in this case too.

It will still be disabled implicitly for non-EAGLE/MTP types or when padded drafter batch is disabled.

This should only be merged once #30495 is merged.

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Enables async scheduling by default when using compatible speculative decoding, with clearer gating and messaging.

• Default-on when speculative_config.method is EAGLE/MTP; otherwise disabled with warnings
• Explicitly disables when disable_padded_drafter_batch=True, pipeline_parallel_size > 1, or executor backend lacks support
• Tightens validation and updates error/warning strings for incompatibilities

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Note

Enables async_scheduling by default when using compatible speculative decoding, with clearer gating and messaging.

• Default-on when speculative_config.method is in EagleModelTypes; otherwise disabled with warning_once and scoped messages
• Explicitly disables for pipeline_parallel_size > 1, disable_padded_drafter_batch=True, or unsupported executor backends, with improved error/warning text
• Replaces several logger.warning calls with logger.warning_once(..., scope="local") to reduce log spam
• Minor string cleanup for incompatibility errors

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[gemini-code-assist]

Code Review

This pull request enables asynchronous scheduling by default when using speculative decoding with EAGLE/MTP methods. The changes correctly update the logic in VllmConfig.__post_init__ to no longer disable async scheduling by default for this configuration. The conditions for disabling async scheduling are now correctly limited to non-EAGLE/MTP speculative decoding methods or when disable_padded_drafter_batch is enabled. Additionally, an error message related to disable_padded_drafter_batch has been improved for clarity and correctness, removing a typo and repetition. The changes are logical and well-aligned with the goal of improving performance by enabling async scheduling in more scenarios. I have not found any issues of high or critical severity.

[mgoin]

Matches my expectation but I'll let @benchislett @LucasWilkinson @MatthewBonanni sign off before merge

[yewentao256]

Thanks for the work! Could you also add a lm_eval to show the acc is correct?

[njhill]

Thanks for the work! Could you also add a lm_eval to show the acc is correct?

@yewentao256 actually this test already checks for precise output match:

vllm/tests/v1/e2e/test_async_scheduling.py

Line 98 in 308feab

def test_with_spec_decoding(sample_json_schema, monkeypatch: pytest.MonkeyPatch):

[MatthewBonanni]

LGTM! We should wait for @benchislett to weigh in though

[benchislett]

LGTM

[yewentao256]

LGTM, thanks!