[Bugfix] Fix DeepSeek V4 MTP HC state handling

[mmangkad]

Purpose

Fix the DeepSeek V4 MTP path after #41536 changed DeepseekV4DecoderLayer.forward to take post_mix, res_mix, and residual. The main model path was updated, but the MTP draft layer still used the old call and failed at startup/profile run. Key error lines:

TypeError: missing required positional argument: post_mix

torch._dynamo.exc.Unsupported: failed to bind arguments when attempting to inline
  Explanation: Argument mismatch when attempting to trace function forward.
  Developer debug context: func='forward' /root/vllm-workspace/vllm/model_executor/models/deepseek_v4.py:1201; args = [<class 'vllm.model_executor.models.deepseek_v4.DeepseekV4DecoderLayer'>]; kwargs = {'positions': TensorVariable(), 'x': TensorVariable(), 'input_ids': ConstantVariable(NoneType: None)}

This updates the MTP call site and fixes the DeepseekV4DecoderLayer.forward return annotation.

Before this fix, DeepSeek V4 with MTP failed to start. After this fix, it starts successfully.

cc @WoosukKwon

[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.

[gemini-code-assist]

Code Review

This pull request updates the DeepSeek-V4 model implementation by modifying the forward method in deepseek_v4.py to return a tuple of four tensors instead of a single tensor. In deepseek_v4_mtp.py, the mtp_block call is updated to unpack these four values, which are then passed to a new hc_post call. I have no feedback to provide as there were no review comments.

[leihuang-sketch]

LGTM

Accuracy is aligned

┌───────────────────┬───────────┬──────────┬──────────┬───────┬─────────┬─────────┐
│ Model             │ Dataset   │ Metric   │ Subset   │   Num │   Score │ Cat.0   │
├───────────────────┼───────────┼──────────┼──────────┼───────┼─────────┼─────────┤
│ DeepSeek-V4-Flash │ gsm8k     │ mean_acc │ main     │  1319 │  0.9689 │ default │
└───────────────────┴───────────┴──────────┴──────────┴───────┴─────────┴─────────┘

2026-05-12 14:13:44 - evalscope - INFO: Skipping report analysis (`analysis_report=False`).
2026-05-12 14:13:44 - evalscope - INFO: Dump report to: ./outputs/20260512_140024/reports/DeepSeek-V4-Flash/gsm8k.json

2026-05-12 14:13:44 - evalscope - INFO:
gsm8k perf table:
Model              Dataset      Num    Avg Lat  Avg TTFT    Avg TPOT      Avg Thpt    Avg In    Avg Out
                                           (s)  (ms)        (ms)           (tok/s)       Tok        Tok
-----------------  ---------  -----  ---------  ----------  ----------  ----------  --------  ---------
DeepSeek-V4-Flash  gsm8k       1319     0.6041  -           -               225.48   582.951    136.215

[WoosukKwon]

Thanks for the PR!