[SD 3.5 Dreambooth LoRA] support configurable training block & layers

[linoytsaban]

This PR adds

• --lora_layers and --lora_blocks to dreambooth lora training script to allow targeting specific blocks & layers.
• This is generally proven to be useful, and specifically for SD3.5 initial thoughts on the matter brought up here - Stable Diffusion 3.5 Large Fine-tuning Tutorial
• some initial results to demonstrate the differences:

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[linoytsaban]

cc @bghira - maybe you explored and have some insights/thoughts to share too 🤗