[Bugfix][GGUF] Accept file-type-only quant types (IQ2_M, IQ3_XS, ...) in remote GGUF model IDs

[Sunt-ing]

Purpose

Fixes #42734.

vllm serve <repo>:UD-IQ2_M (and any repo_id:quant_type reference whose quant is IQ2_M, IQ3_M, IQ3_XS, or MXFP4_MOE) fails with Repo id must use alphanumeric chars...: the whole string is treated as a plain repo id instead of a remote GGUF reference.

Root cause. is_valid_gguf_quant_type() only checks GGMLQuantizationType (the tensor quantization enum), but the quant_type in a repo_id:quant_type reference is a GGUF file type (LlamaFileType) used to select the .gguf file. These are two distinct gguf enums, and IQ2_M/IQ3_M/IQ3_XS/MXFP4_MOE exist only in LlamaFileType — they have no GGMLQuantizationType member, and no valid base after suffix stripping (IQ2_M → IQ2, which doesn't exist). So is_remote_gguf() returns False and the reference is rejected. UD-IQ1_S/UD-IQ1_M work only because IQ1_S/IQ1_M happen to exist in both enums, which is why the check added in #39471 didn't surface this.

This matches the upstream conclusion in ggml-org/llama.cpp#23085: the gguf maintainer confirms this is a downstream (vLLM) issue and that general.file_type must be parsed with LlamaFileType. It also cannot be addressed by adding IQ2_M to GGMLQuantizationType upstream, because GGMLQuantizationType.IQ1_M and LlamaFileType.MOSTLY_IQ2_M both equal 29 in the shared int space.

Fix. Accept either enum in is_valid_gguf_quant_type(): a LlamaFileType file type (members are prefixed MOSTLY_) or a GGMLQuantizationType tensor type. The existing suffix handling for extended names (e.g. Q4_K_M → Q4_K) is preserved. Minimal change, no special-case table, so newly added file types are picked up automatically.

This is orthogonal to #41488 (remote GGUF loading under ModelScope), which handles config/download/MoE-arch for already-recognized quants and does not touch the validation gate; the two are complementary.

Test Plan

• Unit: extend tests/transformers_utils/test_utils.py::TestIsRemoteGGUF to cover file-type-only quants (IQ2_M/IQ3_M/IQ3_XS/MXFP4_MOE), with and without a vendor prefix (UD-), plus negative cases (IQ9_M, NOTATYPE). Each new assertion was confirmed to fail on the unpatched code and pass after the fix.
• E2E: serve a real model through a file-type-only quant reference and generate, confirming the reference now flows past parsing into a working load and generation.

Test Result

Unit tests:

$ pytest tests/transformers_utils/test_utils.py -q
23 passed

E2E — real generation through the remote-ref path this PR unblocks:

model = "bartowski/Qwen2.5-0.5B-Instruct-GGUF:IQ2_M"   # file-type-only quant

INFO ... Resolved architecture: Qwen2ForCausalLM
INFO ... Model loading took 0.31 GiB memory ... load_format=gguf

PROMPT='The capital of France is'  GEN=' Paris. It is the oldest capital city in the world, ...'
PROMPT='Q: 2+2=? A:'               GEN=' 4 B: 5 C: 6 D: 7 ...'

Before this PR, the same reference is rejected at startup with Repo id must use alphanumeric chars....

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[Isotr0py]

LGTM, but given that we're migrating GGUF support (#39612), let's pick it at https://github.com/vllm-project/vllm-gguf-plugin instead.

[Sunt-ing]

LGTM, but given that we're migrating GGUF support (#39612), let's pick it at https://github.com/vllm-project/vllm-gguf-plugin instead.

@Isotr0py Thanks for your review. Opened the same fix in vllm-gguf-plugin as you suggested: vllm-project/vllm-gguf-plugin#19 . Happy to keep this PR as a stopgap until #39612 lands, or close it in favor of the plugin one — whichever you prefer