[MoE Refactor] Turn ChunkingMoERunner into a wrapper so it can be used with any MoERunner subclass.

[bnellnm]

Purpose

Turn ChunkingMoERunner into a wrapper so it can be used with any MoERunner subclass.
Note: _maybe_dispatch + _maybe_combine must be nops when using ChunkingMoERunner

Test Plan

Test Result

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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]

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

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

[gemini-code-assist]

Code Review

This pull request is a significant and well-executed refactoring of the Mixture of Experts (MoE) implementation. Turning ChunkingMoERunner into a wrapper and integrating zero expert functionality directly into FusedMoE and a new ZeroExpertRouter greatly improves modularity and code clarity. The introduction of MoERunner and SharedExperts abstractions are also excellent design choices. I've identified one critical issue where an attribute is accessed before initialization, which would cause a runtime error. A code suggestion is provided to fix this. Apart from that, the changes look solid and are a great improvement to the codebase.

[bnellnm]

combined with #35326