[WIP] mm_utils refactor

[hengtaoguo]

Description

Decouple multimodal_utils.py into centralized router and model-specific functions.

• multimodal/utils.py: Commonly shared basic functions and constant.
• multimodal/processor.py: Centralized router. Routes and manages multimodal data preprocessing, prompt formatting, and input preparation for different model types by re-directing to model-specific processor modules.
• Updated dependencies for models.py/decoders.py/maxengine.py etc for the above changes.

Tests

Unit tests:

python -m unittest tests.unit.multimodal_utils_test
python -m unittest tests.unit.qwen3_omni_layers_test.TestQwen3OmniPreprocessing

End2end tests:

SFT tests:

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 41.98113% with 369 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/MaxText/multimodal/processor_qwen3_omni.py	0.00%	172 Missing ⚠️
src/MaxText/multimodal/processor.py	25.00%	65 Missing and 4 partials ⚠️
src/MaxText/multimodal/processor_llama4.py	70.00%	50 Missing and 10 partials ⚠️
src/MaxText/multimodal/processor_gemma3.py	63.63%	32 Missing ⚠️
...rc/MaxText/input_pipeline/_input_pipeline_utils.py	21.42%	10 Missing and 1 partial ⚠️
src/MaxText/multimodal/utils.py	71.05%	8 Missing and 3 partials ⚠️
src/MaxText/layers/models.py	25.00%	5 Missing and 1 partial ⚠️
src/maxtext/decode.py	42.85%	4 Missing ⚠️
src/MaxText/layers/decoders.py	33.33%	2 Missing ⚠️
src/maxtext/utils/maxtext_utils.py	81.81%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!