Refactor Arctic loading to use AutoWeightsLoader

[lalit10]

Purpose

Refactor ArcticForCausalLM weight loading to use AutoWeightsLoader as part of #15697.

This moves Arctic-specific weight remapping logic from ArcticForCausalLM.load_weights into ArcticModel.load_weights, keeps lm_head handling at the ForCausalLM level, and updates MoE layer detection during loading to use config.moe_layer_frequency instead of hardcoded layer parity.

Test Plan

Local validation in a lightweight macOS dev environment:

• python -m py_compile vllm/model_executor/models/arctic.py
• import ArcticForCausalLM through ModelRegistry
• verify load_weights is implemented on both ArcticModel and ArcticForCausalLM

Test Result

Passed:

  python -m py_compile vllm/model_executor/models/arctic.py

Passed:

python - <<'PY'
from vllm.model_executor.models.registry import ModelRegistry
cls = ModelRegistry._try_load_model_cls("ArcticForCausalLM")
print(cls)
assert cls is not None
PY

Output:
<class 'vllm.model_executor.models.arctic.ArcticForCausalLM'>

Passed:

python - <<'PY'
from vllm.model_executor.models.arctic import ArcticModel, ArcticForCausalLM
print("ArcticModel.load_weights:", ArcticModel.load_weights.__qualname__)
print("ArcticForCausalLM.load_weights:", ArcticForCausalLM.load_weights.__qualname__)
PY

Output:
ArcticModel.load_weights: ArcticModel.load_weights
ArcticForCausalLM.load_weights: ArcticForCausalLM.load_weights

[github-actions]

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[gemini-code-assist]

Code Review

This pull request refactors the Arctic model executor by transitioning weight loading to the AutoWeightsLoader and updating the parameter mapping logic to utilize configuration-defined frequencies for MoE and residual layers. The review feedback identifies a potential logic error in the updated mapping implementation where standard MLP layers may have been omitted and recommends restoring an informational log message regarding weight loading times that was removed during the refactor.

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vllm/model_executor/models/arctic.py (493-522)

The previous logic for creating mlp_params_mapping was incorrect. It was adding mappings for residual_mlp to all layers, regardless of whether they were MoE layers or not. This has been corrected in the new implementation, which correctly identifies MoE layers using self.config.moe_layer_frequency and only adds residual_mlp mappings for MoE layers that have use_residual enabled. However, the old implementation had a bug where it would add mappings for both residual_mlp and block_sparse_moe.mlp for non-MoE layers. The new logic seems to have a similar issue but inverted. It appears you've missed adding the mlp_params_mapping for the standard MLP layers (non-MoE layers). The logic for MoE layers seems correct, but the mappings for the dense MLP layers are missing. I've added them back in the suggestion.

vllm/model_executor/models/arctic.py (539-543)

The informational log message about loading times has been removed. While refactoring is good, this message was helpful for users to understand potential delays. It would be beneficial to retain this logging, perhaps within the AutoWeightsLoader or by re-introducing a logger here if it's specific to Arctic's loading characteristics.

[lalit10]

@DarkLight1337 could you please add the ready label so CI can run? This is my first contribution to vLLM and this PR refactors Arctic loading to use AutoWeightsLoader as part of #15697.

[DarkLight1337]

LGTM, thanks