[Spec Decode] Support single-file checkpoints for target and draft models

[WhatGhost]

Summary

Two small fixes that together let a draft model whose weights are a single file resolve through the speculative decoding config path. Both are places where a condition that is not an error is treated as one, and both sit on that same path, so fixing either on its own changes nothing observable.

The case that motivated them is a GGUF draft head shipped as one file next to the target it drafts for, loaded by an out-of-tree quantization plugin. Neither fix is specific to GGUF: any draft that is not an HF directory hits the same two places.

Relationship to the GGUF plugin PRs

This is the upstream half of GGUF support for this model pair in the out-of-tree plugin, so it is worth reading alongside those PRs. vllm-project/vllm-gguf-plugin#113 (Muse Glimmer multimodal GGUF) carries a monkeypatch that replaces maybe_override_with_speculators so that a .gguf target survives the probe, and vllm-project/vllm-gguf-plugin#115 (the dflash draft head), stacked on it, works around the missing draft hf_config_path by rewriting speculative_config["model"] to the config directory and restoring the weights reference afterwards. Both plugin PRs work today without this one; what they cannot do is stop reaching into vLLM internals to get there. With the two fixes below in, the follow-up plugin PR (vllm-project/vllm-gguf-plugin#117) deletes both workarounds and passes hf_config_path straight through; it stays a draft until this PR lands, since it needs both to exist. The launch command is the same before and after, so removing the workarounds changes nothing user-visible.

The two

1. SpeculativeConfig accepts hf_config_path and forwards it to the draft ModelConfig. ModelConfig has resolved its config and its weights from separate references for a long time, and EngineArgs forwards hf_config_path for the target; the draft had no way to say the same thing. Without it, the only way to point a draft at its config is to rewrite speculative_config["model"] and restore the weights path afterwards, which silently loads the wrong checkpoint if it happens twice.
2. The speculators probe tolerates a reference it cannot read. maybe_override_with_speculators calls get_config_dict purely to check for a speculators_config key; a reference it cannot read is not a speculators model, which is the conclusion it already draws four lines later when the key is absent. Raising instead rejects formats an out-of-tree config parser would have resolved, before that parser runs.

Testing

• Unit tests for each: hf_config_path reaching draft_model_config while the weights reference stays put; the probe returning its inputs unchanged for an unreadable reference.
• End to end with a 2.56B GGUF draft against a 30B GGUF target, via the out-of-tree GGUF plugin, with that plugin's workarounds for both fixes removed so these paths are the only ones in play: 60.8% . Sampling the loading process confirms every weight came from a GGUF file and neither the target's nor the draft's unquantized safetensors was touched.
• The same pair through vllm serve using the command below: the server starts, the draft resolves to its own architecture from hf_config_path while speculative_config["model"] stays pointed at the .gguf file, and speculative decoding is live on the served request (42 of 66 draft tokens accepted).

Usage

Once the plugin PRs are merged, a GGUF target with a GGUF draft head launches as below, with config and tokenizer resolved from the HF directories and every weight read from the .gguf files:

vllm serve /models/Muse-Glimmer-30B-GGUF/Muse-Glimmer-30B-KQuant-Dynamic-Q4_K_XL.gguf \
  --tokenizer /models/Muse-Glimmer-30B \
  --hf-config-path /models/Muse-Glimmer-30B \
  --speculative-config '{"model": "/models/Muse-Glimmer-30B-GGUF/dflash-Muse-Glimmer-30B-Q4_K_M.gguf", "hf_config_path": "/models/Muse-Glimmer-30B-assistant", "num_speculative_tokens": 3}'

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