[Bugfix] Workaround for DeepSeek V4 MTP=1 hang issue

[wzhao18]

Purpose

Currently running DeepSeek V4 MTP=1 will hang. Reproduction:

VLLM_ENGINE_READY_TIMEOUT_S=3600 python3 \
    -m vllm.entrypoints.cli.main \
    serve deepseek-ai/DeepSeek-V4-Pro \
    --trust-remote-code \
    --max-num-batched-tokens 2048 \
    --max-model-len 2048 \
    --max-cudagraph-capture-size 2048 \
    --no-enable-prefix-caching \
    --tensor-parallel-size 8 \
    --enable-expert-parallel \
    --attention_config.use_fp4_indexer_cache=True \
    --kv-cache-dtype fp8 \
    --compilation-config '{"cudagraph_mode":"FULL_AND_PIECEWISE", "custom_ops":["all"]}' \
    --speculative-config '{"method": "mtp", "num_speculative_tokens": 1}' \
    --block-size 256 \
    --enable-auto-tool-choice \
    --tokenizer-mode deepseek_v4 \
    --tool-call-parser deepseek_v4 \
    --no-enable-flashinfer-autotune \
    --reasoning-parser deepseek_v4

python3 InferenceX/utils/bench_serving/benchmark_serving.py --model deepseek-ai/DeepSeek-V4-Pro --backend vllm --base-url [http://0.0.0.0:](http://0.0.0.0:8888/)8000 --dataset-name random --random-input-len 1024 --random-output-len 1024 --random-range-ratio 0.8 --num-prompts 80 --max-concurrency 8 --request-rate inf --ignore-eos --save-result --num-warmups 8 --percentile-metrics ttft,tpot,itl,e2el --trust-remote-code --use-chat-template --dsv4

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

[gemini-code-assist]

Code Review

This pull request introduces the DeepseekV4IndexerMetadataBuilder class, which inherits from DeepseekV32IndexerMetadataBuilder and defines natively_supported_next_n_fp4. It also updates the indexer to return this new builder class through the get_builder_cls static method. I have no feedback to provide as there were no review comments to evaluate.

[wzhao18]

Should be fixed by: #41189