[Doc] Add engine args back in to the docs

[hmellor]

Changes to vLLM:

• Adds requirements necessary to import FlexibleArgumentParser, EngineArgs and AsyncEngineArgs without fully installing vLLM
• Update some imports to be more defensive:
  • Only import for type checking if the class is only used for type hints
  • Delay third party imports when the engine args needs something from a submodule that doesn't require the import

Changes to docs:

• New hook to generate argparse pages in docs/argparse
• New override to enable toc_depth in frontmatter:
  • Allows us to specify generated arguments as headings without polluting the TOC
  • Allows users to link to specific arguments, this was not possible in the Sphinx version of this doc
• Update docs/configuration/engine_args.md to use the generated docs

Follow up:

• Add the server CLI after [frontend] Refactor CLI Args for a better modular integration #20206 is merged

Fixes #18707
FIX #20444

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @hmellor.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[gemini-code-assist]

Summary of Changes

Hello @hmellor, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on improving the documentation for vLLM, particularly around engine arguments. It introduces a new system for automatically generating documentation from argparse configurations, and it also makes some changes to the vLLM codebase to support this new documentation system and to clean up some deprecated features.

Highlights

• Documentation: Adds a new hook to generate argparse pages in docs/argparse, automating the documentation of command-line arguments.
• Documentation: Introduces a new override to enable toc_depth in frontmatter, allowing specification of generated arguments as headings without polluting the table of contents and enabling linking to specific arguments.
• vLLM Changes: Adds requirements necessary to import FlexibleArgumentParser, EngineArgs and AsyncEngineArgs without fully installing vLLM.
• vLLM Changes: Updates imports to be more defensive by only importing for type checking if the class is only used for type hints and delaying third-party imports when the engine args needs something from a submodule that doesn't require the import.
• vLLM Changes: Stops using StoreBoolean in tests, as it's not used by any parsers, paving the way for its complete removal.

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[gemini-code-assist]

Code Review

This pull request introduces automatic generation of documentation for engine arguments, enhancing code quality and maintainability through defensive imports and a well-structured documentation generation script.

[github-actions]

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[hmellor]

Docs build is failing because of some missing mocking (my developement environment didn't have vLLM installed, but it had been installed before so some files which are only generated on install were present).

I'll look into fixing it soon.

[hmellor]

Preview: https://vllm--20674.org.readthedocs.build/en/20674/configuration/engine_args.html

Build time: 6m42 (🎉)

[DarkLight1337]

For triple backtick-formatted strings, the subsequent lines should not be indented at all

[DarkLight1337]

We should also update the Server Arguments doc

[hmellor]

We should also update the Server Arguments doc

This is listed as a follow up task in the PR description, it depends on another PR that cleans up the server CLI

[DarkLight1337]

LGTM, thanks a lot for working on this!