Fix Qwen3-Omni inference when mixing video and image inputs in one batch
Author: BakerBunker
Pull request: yes

# What does this PR do?

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Fixes https://github.com/QwenLM/Qwen3-Omni/issues/91 
Fixes https://github.com/QwenLM/Qwen3-Omni/issues/93
Fixes https://github.com/huggingface/transformers/issues/41680


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Comments:
This change is intended to make the code more consistent with Qwen3-VL. 
https://github.com/huggingface/transformers/blob/71db0d49e99884566026c140f8b12b61056fa8dc/src/transformers/models/qwen3_vl_moe/modeling_qwen3_vl_moe.py#L1298-L1339

Additionally, with the current approach, `embed_joint` would be `hidden_size` times bigger than this pr, requiring the creation of a large tensor, which is memory-inefficient.

Add `router_aux_loss_coef` to `Qwen3OmniMoeThinkerForConditionalGeneration`, fixes https://github.com/huggingface/transformers/issues/41680

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: qwen3_omni_moe

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