added training script for instruct-pix2pix-sdxl

[mzeynali]

What does this PR do?
This PR adds comprehensive documentation for training InstructPix2Pix-SDXL with LoRA adapters on diffusers/sdxl-instructpix2pix-768. The new README_lora_sdxl.md provides:

Key Additions:
Complete training guide for LoRA-based InstructPix2Pix SDXL fine-tuning
Quick start examples with both HuggingFace datasets and custom local datasets
LoRA configuration recommendations for different use cases (fast training, balanced, high quality)
Advanced training options including:
Multi-GPU distributed training setup
Checkpoint resumption
Custom VAE integration for improved stability
Comprehensive inference examples with detailed parameter explanations
Troubleshooting section addressing common issues (OOM errors, NaN loss, quality issues)
LoRA vs full fine-tuning comparison to help users choose the right approach
Documentation Improvements:
Clear argument reference organized by category (Model/Data, Training, LoRA, etc.)
Practical tips for memory optimization
Production-ready inference code snippets
Citations for InstructPix2Pix and SDXL papers
This documentation complements the existing examples/instruct_pix2pix/train_instruct_pix2pix_lora_sdxl.py script and makes it significantly easier for users to:

Get started with LoRA training quickly
Understand and configure LoRA hyperparameters
Troubleshoot common issues
Deploy trained models for inference

Who can review?
@sywangyi - Training examples and documentation

@sayakpaul - Documentation review

[sayakpaul]

Hi, thanks for wanting to contribute this. But I am not if we need these examples right now.

[mzeynali]

Hi, thanks for wanting to contribute this. But I am not if we need these examples right now.

Hello dear, thanks for replying. Why? What's the problem?