Implement custom MoE HashRouter, TopKRouter, and sqrtsoftplus

[parambole]

Description

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Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 12.96296% with 47 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/layers/moe.py	12.96%	47 Missing ⚠️

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