feat: add mtp ut case

[ZhaoJiangJiang]

What this PR does / why we need it?

Fix mtp mode ut

Does this PR introduce any user-facing change?

Nothing

How was this patch tested?

This can be tested in the same way as a unit test.

• vLLM version: v0.10.0
• vLLM main: vllm-project/vllm@14a5d90

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@9260910). Learn more about missing BASE report.
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2166   +/-   ##
=======================================
  Coverage        ?   76.26%           
=======================================
  Files           ?      117           
  Lines           ?    13265           
  Branches        ?        0           
=======================================
  Hits            ?    10116           
  Misses          ?     3149           
  Partials        ?        0           

Flag	Coverage Δ
unittests	76.26% <100.00%> (?)

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