[bugfix] fix qwen3_tts

[Jintao-Huang]

#9715
QwenLM/Qwen3-TTS#278

[gemini-code-assist]

Code Review

This pull request refactors the loss calculation for the Qwen TTS model by removing the labels argument from the talker forward pass in swift/model/models/qwen.py and manually computing the cross-entropy loss in swift/template/templates/qwen.py. Feedback highlights a critical bug in qwen.py where the wrong hidden states layer is accessed and the batch dimension is discarded, as well as an opportunity in templates/qwen.py to scale the loss by num_items_in_batch for correct gradient scaling in distributed training.

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