[PluggableLayer][1/N] Define PluggableLayer (Fix ci)

[whx-sjtu]

Reapplies #32331 that was reverted in #32725.

Purpose

First implementation of RFC #23786: Define PluggableLayer and apply to MLA as an example.

Test Plan

All ci should pass.

Test Result

All ci should pass.

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[mergify]

Documentation preview: https://vllm--32744.org.readthedocs.build/en/32744/

[whx-sjtu]

@ProExpertProg @robertgshaw2-redhat Can you open full ci test for me to find errors please?

[gemini-code-assist]

Code Review

This pull request introduces the PluggableLayer abstraction, a significant refactoring aimed at improving modularity for out-of-tree (OOT) layers. The changes are well-structured, moving op_registry to a global scope and applying PluggableLayer to MultiHeadLatentAttentionWrapper as a practical example. My review focuses on improving the robustness and clarity of the new PluggableLayer and refactored CustomOp APIs. I've identified several areas where the implementation could be made less error-prone and more intuitive for developers, particularly around class instantiation and OOT registration. The docstring for the example usage in mla.py could also be clearer to guide users correctly.

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[ProExpertProg]

CI running

[wangxiyuan]

@whx-sjtu CI failure looks easy to fix. Just need to address the CustomOp changes in test code.

[whx-sjtu]

@whx-sjtu CI failure looks easy to fix. Just need to address the CustomOp changes in test code.

Yes. Working on it now.