[Doc] Fix vLLMEngine Doc Page

[ywang96]

The documentation page is down again for vLLMEngine :(

I'm not sure why but this time we necessarily need to include both transformers and py-cpuinfo in requirements-docs.txt to make it work. See https://vllm--3791.org.readthedocs.build/en/3791/dev/engine/llm_engine.html for a working version.

[simon-mo]

I think it's time to do a proper fix! The root cause of this problem is that readthedocs's builder only install requirements-doc.txt but our CI's doc build have all test dependencies. A good test would be change the existing doc build in CI step to start with a clean python environment (not sure how easy it is to achieve that, maybe just run in a new virtualenv), and only install these packages.

Additionally, the problem can be addressed by using mocking in the conf.py instead of installing heavy weight packages.

[ywang96]

I think it's time to do a proper fix! The root cause of this problem is that readthedocs's builder only install requirements-doc.txt but our CI's doc build have all test dependencies. A good test would be change the existing doc build in CI step to start with a clean python environment (not sure how easy it is to achieve that, maybe just run in a new virtualenv), and only install these packages.

Additionally, the problem can be addressed by using mocking in the conf.py instead of installing heavy weight packages.

Make sense - it baffles me why mocking didn't work but I will look into it.