Add features to the Dreambooth LoRA SDXL training script (#5508)

* Additions:
- support for different lr for text encoder
- support for Prodigy optimizer
- support for min snr gamma
- support for custom captions and dataset loading from the hub

* adjusted --caption_column behaviour (to -not- use the second column of the dataset by default if --caption_column is not provided)

* fixed --output_dir / --model_dir_name confusion

* added --repeats, --adam_weight_decay_text_encoder
+ some fixes

* Update examples/dreambooth/train_dreambooth_lora_sdxl.py

Co-authored-by: Patrick von Platen <[email protected]>

* Update examples/dreambooth/train_dreambooth_lora_sdxl.py

Co-authored-by: Patrick von Platen <[email protected]>

* Update examples/dreambooth/train_dreambooth_lora_sdxl.py

Co-authored-by: Patrick von Platen <[email protected]>

* - import compute_snr from diffusers/training_utils.py
- cluster adamw together
- when using 'prodigy', if --train_text_encoder == True and --text_encoder_lr != --learning rate, changes the lr of the text encoders optimization params to be --learning_rate (otherwise errors)

* shape fixes when custom captions are used

* formatting and a little cleanup

* code styling

* --repeats default value fixed, changed to 1

* bug fix - removed redundant lines of embedding concatenation when using prior_preservation (that duplicated class_prompt embeddings)

* changed dataset loading logic according to the following usecases (to avoid unnecessary dependency on datasets)-
1. user provides --dataset_name
2. user provides local dir --instance_data_dir that contains a metadata .jsonl file
3. user provides local dir --instance_data_dir that contains only images
in cases [1,2] we import datasets and use load_dataset method, in case [3] we process the data same as in the original script setting

* styling fix

* arg name fix

* adjusted the --repeats logic

* -removed redundant arg and 'if' when loading local folder with prompts
-updated readme template
-some default val fixes
-custom caption tests

* image path fix for readme

* code style

* bug fix

* --caption_column arg

* readme fix

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

Author: linoytsaban