[spec decode] Consolidate speculative decode method name for MTP

[zixi-qi]

Purpose

Stack on top of #25221

Currently there are multiple duplicated speculative decode method names for MTP: "deepseek_mtp", "ernie_mtp", "qwen3_next_mtp", "mimo_mtp". This PR consolidates them into one "mtp" method so that we do not need to keep appending this list when creating new models with MTP

Test Plan

Ran unit tests and e2e tests

Test Result

• unit tests

(vllm) ubuntu@209-20-159-113:~/vllm$ pytest tests/v1/e2e/test_spec_decode.py::test_mtp_correctness -v
==================================================================================================== test session starts =====================================================================================================
platform linux -- Python 3.12.11, pytest-8.4.2, pluggy-1.6.0 -- /home/ubuntu/vllm/.venv/bin/python
cachedir: .pytest_cache
rootdir: /home/ubuntu/vllm
configfile: pyproject.toml
plugins: anyio-4.10.0
collected 3 items                                                                                                                                                                                                            

tests/v1/e2e/test_spec_decode.py::test_mtp_correctness[FLASH_ATTN_VLLM_V1-mtp] PASSED                                                                                                                                  [ 33%]
tests/v1/e2e/test_spec_decode.py::test_mtp_correctness[TRITON_ATTN_VLLM_V1-mtp] SKIPPED (TRITON_ATTN_VLLM_V1 does not support multi-token MTP spec decode on current platform)                                         [ 66%]
tests/v1/e2e/test_spec_decode.py::test_mtp_correctness[TREE_ATTN-mtp] SKIPPED (MTP does not support tree-based speculative decoding)                                                                                   [100%]

====================================================================================================== warnings summary ======================================================================================================
<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyPacked has no __module__ attribute

<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyObject has no __module__ attribute

tests/v1/e2e/test_spec_decode.py::test_mtp_correctness[FLASH_ATTN_VLLM_V1-mtp]
tests/v1/e2e/test_spec_decode.py::test_mtp_correctness[FLASH_ATTN_VLLM_V1-mtp]
  /home/ubuntu/.local/share/uv/python/cpython-3.12.11-linux-x86_64-gnu/lib/python3.12/multiprocessing/popen_fork.py:66: DeprecationWarning: This process (pid=137287) is multi-threaded, use of fork() may lead to deadlocks in the child.
    self.pid = os.fork()

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
==================================================================================== 1 passed, 2 skipped, 4 warnings in 65.78s (0:01:05) =====================================================================================
sys:1: DeprecationWarning: builtin type swigvarlink has no __module__ attribute
(vllm) ubuntu@209-20-159-113:~/vllm$ pytest tests/v1/spec_decode/test_mtp.py -v
==================================================================================================== test session starts =====================================================================================================
platform linux -- Python 3.12.11, pytest-8.4.2, pluggy-1.6.0 -- /home/ubuntu/vllm/.venv/bin/python
cachedir: .pytest_cache
rootdir: /home/ubuntu/vllm
configfile: pyproject.toml
plugins: anyio-4.10.0
collected 4 items                                                                                                                                                                                                            

tests/v1/spec_decode/test_mtp.py::test_mtp_load_model_unified PASSED                                                                                                                                                   [ 25%]
tests/v1/spec_decode/test_mtp.py::test_mtp_propose_returns_hidden_states[1-FLASH_ATTN_VLLM_V1] PASSED                                                                                                                  [ 50%]
tests/v1/spec_decode/test_mtp.py::test_mtp_propose_returns_hidden_states[1-TRITON_ATTN_VLLM_V1] SKIPPED (TRITON_ATTN_VLLM_V1 does not support multi-token spec decode on current platform)                             [ 75%]
tests/v1/spec_decode/test_mtp.py::test_mtp_propose_returns_hidden_states[1-TREE_ATTN] SKIPPED (MTP does not support tree-based speculative decoding)                                                                   [100%]

====================================================================================================== warnings summary ======================================================================================================
<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyPacked has no __module__ attribute

<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: builtin type SwigPyObject has no __module__ attribute

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
========================================================================================= 2 passed, 2 skipped, 2 warnings in 15.35s ==========================================================================================

• e2e test

VLLM_USE_V1=1 python examples/offline_inference/spec_decode.py --num_spec_tokens 5 --num_prompts 80 --dataset-name hf --dataset-path philschmid/mt-bench --method mtp --model-dir XiaomiMiMo/MiMo-7B-Base --num-spec-tokens 2 --tp 1 --enforce-eager

--------------------------------------------------
total_num_output_tokens: 19758
num_drafts: 9626
num_draft_tokens: 19252
num_accepted_tokens: 10091
mean acceptance length: 2.05
--------------------------------------------------
acceptance at token 0: 0.87
acceptance at token 1: 0.18

---

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.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request consolidates various MTP speculative decoding method names into a unified "mtp" method. The implementation of this consolidation in the configuration and core logic appears correct. However, I've identified a few areas for improvement, primarily in the tests, which are still using deprecated method names, and a misleading warning message. My review includes suggestions to align the tests with the new consolidated method and to clarify the warning message for better user experience.

[luccafong]

LGTM! thanks for consolidating the codes.