Some wrong in sd3's lora training script

### Describe the bug

https://github.com/huggingface/diffusers/blob/e8aacda762e311505ba05ae340af23b149e37af3/examples/research_projects/sd3_lora_colab/train_dreambooth_lora_sd3_miniature.py#L717

the transformer before accelerator.prepare should not convert to fp16 in mix precision;
it will broken the grad precision and result in worse training result

### Reproduction

see the "/examples/research_projects/sd3_lora_colab/train_dreambooth_lora_sd3_miniature.py"

### Logs

```shell

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### System Info

diffusers 0.31.0

### Who can help?

_No response_