Register Triton implementation as a custom op to fix the Mixer2 gated RMS norm lowering issue.

[chaojun-zhang]

Purpose

1. Add a test for Mixer2 gated RMS norm lowering
2. Register Triton implementation as a custom op to fix the lowering issue.

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.