Error when I'm trying to train a Flux lora with train_dreambooth_lora_flux_advanced

### Describe the bug

Hi! I'm trying to train my lora model with [train_dreambooth_lora_flux_advanced](https://github.com/huggingface/diffusers/blob/main/examples/advanced_diffusion_training/train_dreambooth_lora_flux_advanced.py) script.

When I'm trying to train my model with prior preservation tag I give an error.

How can I fix it?

### Reproduction

```bash
accelerate launch train_dreambooth_lora_flux_advanced.py \
--pretrained_model_name_or_path="black-forest-labs/FLUX.1-dev" \
--dataset_name="./ds5" \
--instance_prompt="1boy, 1girl" \
--validation_prompt="1boy, 1girl" \
--class_prompt="1boy, 1girl" \
--num_class_images=200 \
--with_prior_preservation \
--class_data_dir="./cdi" \
--output_dir="crtr-SDXL-LoRA" \
--caption_column="text" \
--mixed_precision="bf16" \
--prior_generation_precision="bf16" \
--resolution=1024 \
--train_batch_size=8 \
--repeats=1 \
--gradient_accumulation_steps=8 \
--gradient_checkpointing \
--learning_rate=1.0 \
--optimizer="prodigy"\
--lr_scheduler="constant" \
--lr_warmup_steps=0 \
--rank=64 \
--num_train_epochs=200 \
--validation_epochs=100 \
--center_crop \
--adam_beta2=0.99 \
--adam_weight_decay=0.01 \
--allow_tf32
```

### Logs

```shell
Traceback (most recent call last):
File "/workspace/train_dreambooth_lora_flux_advanced.py", line 2423, in
main(args)
File "/workspace/train_dreambooth_lora_flux_advanced.py", line 2213, in main
(weighting.float() * (model_pred_prior.float() - target_prior.float()) ** 2).reshape(
~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
RuntimeError: The size of tensor a (16) must match the size of tensor b (8) at non-singleton dimension 0
```

### System Info

Diffusers 0.33
CUDA 12.9
Torch 2.7

Docker image
nvcr.io/nvidia/pytorch:25.04-py3

### Who can help?

@sayakpaul

Start by running the provided accelerate command with train_dreambooth_lora_flux_advanced.py and inspect main() near line 2213, where the prior-preservation loss raises the tensor-size error. Trace the batch and weighting shapes through that path, then verify that prior-preservation training completes without the reported mismatch.

Scritto dal modello di indicizzazione a partire dal testo della issue.