[Bugfix] fix pp for llama4

[luccafong]

This PR fixes PP for llama4 (#16385)

Loading language model with architecture to support PP verification and correct the prefix to separate model loading weights for language model and rest of the models so both PP=0 and PP=1 can work.

VLLM_DISABLE_COMPILE_CACHE=1 python -m vllm.entrypoints.openai.api_server --model $LLAMA_DIR --served-model-name maverick  --max-model-len 20000 --tensor-parallel 4 --pipeline-parallel-size 2 --gpu-memory-utilization 0.95  --port 8000 

curl http://localhost:8000/v1/completions \
    -H "Content-Type: application/json" \
    -d '{
        "model": "maverick",
        "prompt": "San Francisco is a",
        "max_tokens": 7,
        "temperature": 0
    }'

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value|   |Stderr|
|-----|------:|----------------|-----:|-----------|---|----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.930|±  |0.0181|
|     |       |strict-match    |     5|exact_match|↑  |0.935|±  |0.0175|

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[houseroad]

Should we use Llama4ForCausalLM?

[luccafong]

Llama4ForCasualLm is not registered architecture, we should avoid using that which requires adding a lot of hacks as in the initial PR

[ywang96]

Probably a dumb question, but since _initialize_model is already pointing to model_class=Llama4ForCausalLM, why do we need to override the architectures here to LlamaForCausalLM?

vllm/vllm/model_executor/model_loader/loader.py

Lines 114 to 123 in 8cac35b

	def _initialize_model(
	vllm_config: VllmConfig,
	*,
	prefix: str = "",
	model_class: Optional[type[nn.Module]] = None,
	) -> nn.Module:
	"""Initialize a model with the given configurations."""
	model_config = vllm_config.model_config
	if model_class is None:
	model_class, _ = get_model_architecture(model_config)

[luccafong]

during __post__init__ of the hf config when we call replace inside with_hf_config function

vllm/vllm/config.py

Line 3781 in a018e55

return replace(self, model_config=model_config)

, it will check if the architecture is supporting PP or not via

vllm/vllm/config.py

Line 3790 in a018e55

self.model_config.verify_with_parallel_config(self.parallel_config)

, where None architecture will raise an issue as in #16385 in below code

vllm/vllm/model_executor/models/registry.py

Lines 441 to 442 in a018e55

	normalized_arch = list(
	filter(lambda model: model in self.models, architectures))

[houseroad]

Wondering why this issue was not triggered before?

[luccafong]

language_model.lm_head can also be loaded by the parent model weight loader if not PP enabled, but for PP, as we split the weights into 2 parts, the lm_head is missing in PP=0 so it will raise an issue weight not found. while in llama4.py model loading we have logic handling is_pp_missing_parameter to avoid the exception

[houseroad]

Thanks for the fix.