Create initial tpu-post-train installation option

[bvandermoon]

Description

• Create initial installation option for installing tpu-post-train dependencies
• To get started, the generated requirements currently match the base tpu ones exactly. We will update them to the correct post-train dependencies after this

Tests

uv pip install -e .[tpu-post-train] --resolution=lowest

This installation option is not included in PR tests at this point. Just getting the initial setup, it is not used anywhere so it shouldn't impact other places

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• 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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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