examples/dreambooth: fix missing `weighting` chunk when using prior preservation in Flux and SD3 LoRA training (#13743)
* examples/dreambooth: chunk weighting tensor alongside model_pred and target when using prior preservation (flux LoRA)
* examples/dreambooth: chunk weighting tensor alongside model_pred and target when using prior preservation (SD3 LoRA)
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Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>