[Model] Use AutoWeightsLoader for MiMo

[bittoby]

Purpose

Part of #15697.

This PR refactors MiMoForCausalLM to use AutoWeightsLoader. Mirrors the pattern used by qwen2.py and the recently merged #41492 (Step3Text) / #41448 (LongCat Flash).

Previously MiMoModel.load_weights was a near-duplicate of Qwen2Model.load_weights with one extra branch (if "mtp_layers" in name: continue) to keep MTP-only weights out of the main model. That override is now removed — MiMoModel inherits Qwen2Model.load_weights directly — and MiMoForCausalLM.load_weights delegates through AutoWeightsLoader with skip_substrs=["mtp_layers"], matching the pattern used in deepseek_v4.py (AutoWeightsLoader(self, skip_substrs=["mtp."])).

The standard skip_prefixes=["lm_head."] is applied when config.tie_word_embeddings is set, matching Qwen2ForCausalLM.

This is a refactor only and does not change model architecture or inference behavior. The MTP draft path (MiMoMTP in mimo_mtp.py) is unaffected — it has its own loader and continues to consume the model.mtp_layers.* weights that this loader skips.

Net diff: +10 / -61 lines in vllm/model_executor/models/mimo.py.

Test Plan

Local validation:

• python -m py_compile vllm/model_executor/models/mimo.py
• import MiMoForCausalLM through ModelRegistry
• verify load_weights on MiMoModel is inherited from Qwen2Model and MiMoForCausalLM.load_weights delegates via AutoWeightsLoader with the mtp_layers skip
• ruff check / ruff format --check

CI is expected to run the GPU initialization test:

CUDA_VISIBLE_DEVICES=0 python -m pytest \
  tests/models/test_initialization.py::test_can_initialize_large_subset \
  -q -k MiMoForCausalLM -s --tb=short

Test Result

Passed:

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

Passed:

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

Output:

<class 'vllm.model_executor.models.mimo.MiMoForCausalLM'>

Passed:

python - <<'PY'
from vllm.model_executor.models.mimo import MiMoModel, MiMoForCausalLM
print("MiMoModel.load_weights:", MiMoModel.load_weights.__qualname__)
print("MiMoForCausalLM.load_weights:", MiMoForCausalLM.load_weights.__qualname__)
import inspect
src = inspect.getsource(MiMoForCausalLM.load_weights)
print("Outer delegates via AutoWeightsLoader:", "AutoWeightsLoader" in src)
print("Skips mtp_layers:", "mtp_layers" in src)
PY

Output:

MiMoModel.load_weights: Qwen2Model.load_weights
MiMoForCausalLM.load_weights: MiMoForCausalLM.load_weights
Outer delegates via AutoWeightsLoader: True
Skips mtp_layers: True

Passed:

ruff check vllm/model_executor/models/mimo.py
ruff format --check vllm/model_executor/models/mimo.py

Output:

All checks passed!
1 file already formatted

GPU initialization test deferred to CI — submitter has no local CUDA device. The change is Python-only and does not touch any C++ kernels.

AI assistance

This change was AI-assisted (Claude Code). The submitter reviewed every changed line, walked through the AutoWeightsLoader semantics (including how skip_substrs filters at the prefix-grouping level), and validated the structural changes locally.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[github-actions]

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[gemini-code-assist]

Code Review

This pull request refactors the weight loading mechanism in vllm/model_executor/models/mimo.py by replacing a verbose, manual load_weights implementation with the AutoWeightsLoader utility. This change simplifies the codebase while preserving specific logic for skipping mtp_layers and handling tied word embeddings. I have no feedback to provide.

[bittoby]

@DarkLight1337 Ready to review.

[DarkLight1337]

Can you run a lm-eval benchmark to verify the model correctness? Since you changed the semantics of load_weights a bit unlike in your other PR