[Misc][Refactor] Separate router from FusedMoE class

[bnellnm]

Purpose

Separate existing router logic into a standalone class DefaultFusedMoERouter.

Needs #30519

Test Plan

Test Result

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[gemini-code-assist]

Code Review

This pull request refactors the Mixture-of-Experts (MoE) routing logic by introducing an abstract FusedMoERouter class and a concrete DefaultFusedMoERouter implementation. The FusedMoERouter encapsulates all expert selection parameters and methods, which were previously part of the FusedMoE layer. The FusedMoE layer now instantiates a DefaultFusedMoERouter and delegates expert selection to it. Consequently, various quantization methods' apply functions have been updated to accept and utilize this new router object for expert selection. Additionally, EPLB (Expert Placement Load Balancing) state variables have been consolidated into a new EplbLayerState dataclass. A review comment highlights a RecursionError in the DefaultFusedMoERouter's routing_method_type property, suggesting it should return the private _routing_method_type attribute instead of calling itself.

[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

[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

[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

[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