Cannot re-initialize CUDA in forked subprocess. To use CUDA with multiprocessing, you must use the 'spawn' start method
### Describe the bug

1. I ran dataset mapping with `num_proc=6` in it and got this error:
`RuntimeError: Cannot re-initialize CUDA in forked subprocess. To use CUDA with multiprocessing, you must use the 'spawn' start method`

I can't actually find a way to run multi-GPU dataset mapping. Can you help?

### Steps to reproduce the bug

1. Rund SDXL training with `num_proc=6`: https://github.com/huggingface/diffusers/blob/main/examples/text_to_image/train_text_to_image_sdxl.py

### Expected behavior

Should work well

### Environment info

6x A100 SXM, Linux