Remove masks and update gmm function in moe.py

[NuojCheng]

Description

This PR cherrypicks proposed changes from @RissyRan in here, and do following things

• Update gmm function in moe.py and add group_offset input
• Remove unnecessary masks to further extend performance

FIXES: b/519220305

Tests

Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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