Refactor: extract shared post-training hooks and update SFT implementation

[igorts-git]

Description

Generalize the SFT hooks such that they can be used in DPO as well.

We move the shared code into src/maxtext/trainers/post_train/hooks.py while the SFT-specific overrides reside in the SFT sub-directory.

This PR is a pre-requisite for #3668 (DPO implementation)

Tests

Test run of a local SFT training.
Ran the unit tests, including logits comparison test.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• 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

❌ Patch coverage is 73.38710% with 33 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/trainers/post_train/hooks.py	72.03%	29 Missing and 4 partials ⚠️

📢 Thoughts on this report? Let us know!