[spec decode] Add unit tests and e2e test for MTP inference

[zixi-qi]

Purpose

Add test coverage for MTP inference

Test Plan

Ran the newly added tests

Test Result

pytest tests/v1/e2e/test_spec_decode.py::test_mtp_correctness -v

tests/v1/e2e/test_spec_decode.py::test_mtp_correctness[FLASH_ATTN_VLLM_V1-mimo_mtp] PASSED                                                                                                                                         [ 33%]
tests/v1/e2e/test_spec_decode.py::test_mtp_correctness[TRITON_ATTN_VLLM_V1-mimo_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-mimo_mtp] SKIPPED (MTP does not support tree-based speculative decoding)                                                                                          [100%]
========================================================================================== 1 passed, 2 skipped, 4 warnings in 67.84s (0:01:07) ===========================================================================================
sys:1: DeprecationWarning: builtin type swigvarlink has no __module__ attribute

pytest tests/v1/spec_decode/test_mtp.py -v

tests/v1/spec_decode/test_mtp.py::test_mtp_load_model_unified PASSED                                                                                                                                                               [ 14%]
tests/v1/spec_decode/test_mtp.py::test_mtp_propose_returns_hidden_states[1-FLASH_ATTN_VLLM_V1] PASSED                                                                                                                              [ 28%]
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)                                         [ 42%]
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)                                                                               [ 57%]
tests/v1/spec_decode/test_mtp.py::test_mtp_propose_returns_hidden_states[4-FLASH_ATTN_VLLM_V1] PASSED                                                                                                                              [ 71%]
tests/v1/spec_decode/test_mtp.py::test_mtp_propose_returns_hidden_states[4-TRITON_ATTN_VLLM_V1] SKIPPED (TRITON_ATTN_VLLM_V1 does not support multi-token spec decode on current platform)                                         [ 85%]
tests/v1/spec_decode/test_mtp.py::test_mtp_propose_returns_hidden_states[4-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
=============================================================================================== 3 passed, 4 skipped, 2 warnings in 19.53s ================================================================================================

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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.
• (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 adds unit and end-to-end tests for MTP (Multi-Token Prediction) speculative decoding, specifically for the MiMo model. The new tests are well-structured and follow existing patterns. However, I've identified a critical contradiction between the test configurations and a comment in the model's implementation regarding the number of supported speculative tokens. This needs to be resolved to ensure the tests are valid and correctly verify the intended functionality.

[zixi-qi]

Changes from this PR has been moved to #25232