Backend support

SD.Next supports two main backends: Diffusers and Original:

• Diffusers: Based on new Huggingface Diffusers implementation
  Supports all models listed below
  This backend is set as default for new installations
• Original: Based on LDM reference implementation and significantly expanded on by A1111
  This backend and is fully compatible with most existing functionality and extensions written for A1111 SDWebUI
  Supports SD 1.x and SD 2.x models
  All other model types such as SD-XL, LCM, Stable Cascade, PixArt, Playground, Segmind, Kandinsky, etc. require backend Diffusers