DPO/ORPO example notebooks

[igorts-git]

Description

This PR contains DPO and ORPO example notebooks.
These notebooks will also serve as integration tests for DPO.

Tests

Ran the notebooks manually.

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 58.03571% with 47 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/utils/maxtext_utils.py	20.93%	34 Missing ⚠️
src/maxtext/common/metric_logger.py	0.00%	7 Missing and 2 partials ⚠️
src/maxtext/input_pipeline/hf_data_processing.py	57.14%	1 Missing and 2 partials ⚠️
src/maxtext/input_pipeline/dpo_utils.py	98.11%	0 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!