feat: migrate models.py to use nnx

[mesakhcienet]

Description

Enable generalized usage of nnx class Transformer

Model and logs:

gs://mesa-maxtext-bucket/modelspy-nnx-porting/

Profiling:

• before migration : https://xprof.corp.google.com/overview_page/mesakh-15535109900813885609
• after migration : https://xprof.corp.google.com/overview_page/mesakh-2106845934401945903

Tests

Please describe how you tested this change, and include any instructions and/or
commands to reproduce.

Checklist

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• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.