Install vllm from source to make doctest passed

[Yikun]

Related issue: #2742

• vLLM version: v0.10.1.1
• vLLM main: vllm-project/vllm@3bca396

[github-actions]

👋 Hi! Thank you for contributing to the vLLM Ascend project. The following points will speed up your PR merge:‌‌

• A PR should do only one thing, smaller PRs enable faster reviews.
• Every PR should include unit tests and end-to-end tests ‌to ensure it works and is not broken by other future PRs.
• Write the commit message by fulfilling the PR description to help reviewer and future developers understand.

If CI fails, you can run linting and testing checks locally according Contributing and Testing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.61%. Comparing base (4c90fa7) to head (4ff0fa9).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2769      +/-   ##
==========================================
- Coverage   72.99%   72.61%   -0.39%     
==========================================
  Files         153      153              
  Lines       21331    21278      -53     
==========================================
- Hits        15571    15451     -120     
- Misses       5760     5827      +67     

Flag	Coverage Δ
unittests	72.61% <ø> (-0.39%)	⬇️

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