Add post training tests to CI

[dipannita08]

Description

This PR:

• Adds cpu and tpu post training tests to Github CI.
• Moves post training tests into tests/post_training

FIXES: b/491236090

Tests

• CI

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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