[Model] Use merge_by_field_config for MM models (Qwen series)

[DarkLight1337]

Purpose

CLOSE #26149

Test Plan

The tensor schema tests passed locally (Qwen3 series isn't tested in CI yet). I have also run the example scripts for each model (except for Qwen3-Omni because it doesn't have one)

Test Result

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[gemini-code-assist]

Code Review

This pull request refactors the multi-modal input handling for Qwen series models by leveraging the merge_by_field_config mechanism. This is a good improvement as it centralizes the input processing logic, removes redundant code (like _validate_and_reshape_mm_tensor methods) from individual model files, and makes the codebase cleaner and more maintainable. The changes across qwen2_5_omni_thinker.py, qwen2_5_vl.py, qwen2_audio.py, qwen2_vl.py, qwen3_omni_moe_thinker.py, qwen3_vl.py, and qwen_vl.py are consistent with this goal.

I've found one issue that needs to be addressed before merging.