[Bugfix] Parse SpeculativeConfig Error

[yyzxw]

Purpose

fix #24888

Test Plan

Test Result

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• The test plan, such as providing test command.
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[gemini-code-assist]

Code Review

This pull request addresses an error in parsing SpeculativeConfig by refactoring how the --speculative-config command-line argument is handled. The changes correctly fix the immediate parsing issue. However, I've identified a critical flaw in the new type check within _is_v1_supported_oracle. The current fix is incomplete as it only validates the configuration when it's a dictionary, but overlooks the case where it's a SpeculativeConfig object. This could lead to incorrect behavior and subsequent runtime failures. I have provided a detailed comment with a suggested code change to make the validation more robust.

[DarkLight1337]

cc @hmellor

[yyzxw]

@hmellor PTAL, thanks!

[hmellor]

It is already parsed as a dict in

vllm/vllm/engine/arg_utils.py

Lines 914 to 919 in 273690a

	# We construct SpeculativeConfig using fields from other configs in
	# create_engine_config. So we set the type to a JSON string here to
	# delay the Pydantic validation that comes with SpeculativeConfig.
	vllm_kwargs["speculative_config"]["type"] = optional_type(json.loads)
	vllm_group.add_argument("--speculative-config",
	**vllm_kwargs["speculative_config"])

Only the second change is necessary

[hmellor]

LGTM! Thanks

[hmellor]

(sorry I messed up DCO on my commit so I force-pushed to replace just my commit with a signed-off one)