[MoE Refactor] Move expert map related code into ExpertMapManager class

[bnellnm]

Purpose

Create ExpertMapManager class to handle all expert map related functionality in the FusedMoE layer.

Test Plan

CI

Test Result

cc @yzong-rh

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[gemini-code-assist]

Code Review

This pull request introduces the ExpertMapManager class to centralize and manage expert ID mappings, placement strategies, and routing tables for MoE layers, refactoring this logic out of layer.py. The feedback identifies critical state management issues in the update and update_expert_map methods, specifically regarding the synchronization of local expert counts when shared experts are present and the need to keep configuration objects in sync during dynamic updates. Additionally, the round-robin routing table logic requires correction to include shared experts to prevent potential out-of-bounds errors in kernels.

[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

[yzong-rh]

Three remaining concerns with ExpertMapManager

1. self.device might throw an error if expert_maps are not initialized (when ep_size == 1 for example).
2. Usage of self.device in _ensure_routing_tables_initialized is redundant thanks to with self.device:.
3. In order to ensure idempotency _ensure_routing_tables_initialized never override already set routing_tables. This could be problematic in update.

[yzong-rh]

Other than that LGTM. Thanks!

[robertgshaw2-redhat]

I think what is confusing about ExpertMap is that it is used ** sometimes **

It would help a lot for this class to explain when and when it is not used

@varun-sundar-rabindranath might have the best view of this

[bnellnm]

IIUC it's used for EP to map from global<->local expert ids. Not all backends/experts require it though. Some all2alls handle it internally and pass that along to the experts.

[mergify]

Hi @bnellnm, the pre-commit checks have failed. Please run:

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pre-commit run --all-files

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Tip

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[mergify]

Hi @bnellnm, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy failing?

mypy is run differently in CI. If the failure is related to this check, please use the following command to run it locally:

# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10