Add Qwen3-VL-4B support

[hengtaoguo]

Description

This PR adds Qwen/Qwen3-VL-4B-Instruct to maxtext:

• Add Qwen3-VL-4B model config and register Qwen/Qwen3-VL-4B-Instruct.
• Wire Qwen3-VL into multimodal config validation, prompt/image preprocessing, embedding fusion, and vision bidirectional masking.
• Add Qwen3-VL vision encoder/projector subclasses reusing the existing Qwen3 Omni vision tower implementation.
• Extend vision RoPE handling to Qwen3 vision models.
• Add HF config, HF shape metadata, parameter mappings, and conversion hooks for Qwen3-VL checkpoint conversion.
• Preserve non-causal-LM HF config architectures during save when no causal LM mapping exists.

Tests

# Checkpoint conversion
python -m maxtext.checkpoint_conversion.to_maxtext src/maxtext/configs/base.yml model_name=qwen3-vl-4b base_output_directory=gs://hengtaoguo-maxtext-logs/checkpoints/qwen3-vl-4b/unscanned/2026-06-24-15-27 scan_layers=false hf_access_token=<your_token> weight_dtype=bfloat16 hardware=cpu skip_jax_distributed_system=True checkpoint_storage_use_ocdbt=False checkpoint_storage_use_zarr3=False --eager_load_method=safetensors --lazy_load_tensors=False

# Decode check
python -m maxtext.inference.decode maxtext/configs/base.yml model_name=qwen3-vl-4b tokenizer_path=Qwen/Qwen3-VL-4B-Instruct tokenizer_type=huggingface load_parameters_path=gs://hengtaoguo-maxtext-logs/checkpoints/qwen3-vl-4b/unscanned/2026-06-24-15-27/0/items per_device_batch_size=1 run_name=ht_test scan_layers=false use_multimodal=true prompt=\'Describe\ this\ image\' image_path=\'tests/assets/test_image.jpg\' max_prefill_predict_length=512 max_target_length=768 ici_tensor_parallelism=4 override_model_config=true attention=\'dot_product\' hf_access_token=<your_token>

# Outputs
Input `<|im_start|>user
<|vision_start|><|image_pad|><|vision_end|>Describe this image<|im_end|>
<|im_start|>assistant
` -> `This image captures a vibrant, sunlit panoramic view of Seattle, Washington, under a clear blue sky. The most iconic landmark, the Space Needle, stands prominently in the center-left of the frame, its distinctive futuristic design rising above the surrounding urban landscape.

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 2.71739% with 179 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...xtext/checkpoint_conversion/utils/param_mapping.py	1.75%	112 Missing ⚠️
...rc/maxtext/checkpoint_conversion/utils/hf_shape.py	0.00%	46 Missing ⚠️
src/maxtext/layers/encoders.py	0.00%	7 Missing ⚠️
src/maxtext/multimodal/processor.py	12.50%	7 Missing ⚠️
src/maxtext/models/qwen3_vl_vision.py	0.00%	3 Missing ⚠️
src/maxtext/checkpoint_conversion/utils/utils.py	0.00%	2 Missing ⚠️
src/maxtext/layers/attentions.py	0.00%	1 Missing ⚠️
src/maxtext/layers/decoders.py	0.00%	1 Missing ⚠️

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