Adds an advanced version of the SD-XL DreamBooth LoRA training script supporting pivotal tuning (huggingface#5883)

* sdxl dreambooth lora training script with pivotal tuning

* bug fix - args missing from parse_args

* code quality fixes

* comment unnecessary code from TokenEmbedding handler class

* fixup

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Co-authored-by: Linoy Tsaban <[email protected]>

Author: linoytsaban