[Doc] Add missing mock import to docs conf.py

[hmellor]

Fixes the issue identified in #6600 (comment)

FIX #6853

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[DarkLight1337]

Thanks for fixing this! Is there a way to cause failures like https://readthedocs.org/projects/vllm/builds/25082568/ to fail the CI so we won't run into such issues again? (This is far from the first time the API reference has been broken by a missing mock import)

[hmellor]

We could use some combination of nitpicky mode and fail on warning?

This is probably something a RTD admin would have to do though

[DarkLight1337]

Seems like we can update .readthedocs.yaml with those options.

https://docs.readthedocs.io/en/stable/config-file/v2.html

Can you test it out in this PR? (It should fail without the mock import)

[hmellor]

@DarkLight1337 I can't seem to get it to fail...

[DarkLight1337]

python -m sphinx -T -W --keep-going -b html -d _build/doctrees -D language=en . $READTHEDOCS_OUTPUT/html

I think it's because of -W --keep-going. Where did that come from?

[hmellor]

That is what fail_on_warning adds. It turns warnings into errors and adds keep going so that it doesn't crash at the first warning.

[DarkLight1337]

Oh, I didn't read the details of the CLI flags and thought it meant to skip the warnings. Hmm, in that case the build should have failed...

[hmellor]

Oh it did fail in AWS https://buildkite.com/vllm/ci-aws/builds/5709#0190ef62-e04f-4fb9-bfb2-bd29b30540e0

[DarkLight1337]

Nice! Let's put the fix back in this PR and get it merged then.

[DarkLight1337]

Ok so it seems that we need nitpicky mode to catch missing mock imports, but doing so would cause a bunch of other errors. Let's fix the docs first and revisit the issue later.