[Feature]: Optimize EPLB Rearrange Experts

[robertgshaw2-redhat]

🚀 The feature, motivation and pitch

• Rearrange experts can take 15s during wide EP (EP=32) setups
• This blocks the server for much too long
• We need some way to improve this. Ideas include (a) making it async, (b) serializing the state, or (c) making things faster with direct optimization

Alternatives

No response

Additional context

No response

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[david6666666]

(a) making it async #22179

[robertgshaw2-redhat]

Thanks @david6666666 !

[github-actions]

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