[flux dreambooth lora training] make LoRA target modules configurable + small bug fix

[linoytsaban]

new feature for the Flux dreambooth lora training script:

• make LoRA target modules configurable through --lora_blocks

• change the current default target modules to not be attention layers only (?)

& small fix to mixed precision training for dreambooth script, as proposed in https://github.com/huggingface/diffusers/pull/9565

[HuggingFaceDocBuilderDev]

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