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

```

### System Info

diffusers 0.31.0

### Who can help?

_No response_

Start with examples/research_projects/sd3_lora_colab/train_dreambooth_lora_sd3_miniature.py around line 717, then reproduce the issue using that script with diffusers 0.31.0. Check the transformer’s handling before accelerator.prepare in mixed-precision training; done means gradient precision is preserved and the training result no longer regresses.

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