[Feature request] Support single-file stable diffusion safetensors models

It would be convenient to be able to load a full model — CLIP, CLIP2, UNet, VAE — from a single file, like how models are distributed on civitai for example.

Start by locating Candle's stable diffusion model-loading entry points and the existing safetensors handling. Determine how CLIP, CLIP2, UNet, and VAE are currently loaded, then define what support for a single-file model from sources such as civitai should cover. Done means a complete model can be loaded from one file, with coverage for the affected components.

Written by the indexing model from the issue text.