Add rank argument to train_dreambooth_lora_sdxl.py

[levi]

What does this PR do?

Adds the LoRa rank argument to the SDXL Dreambooth LoRa training script.

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@sayakpaul

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sayakpaul]

Thanks for your PR. Could you please run make style && make quality and push the changes?

[patrickvonplaten]

Looks good to me! I'll let the expert merge here cc @sayakpaul