limit extension preload scripts to just one file named preload.py

By creating a file called "preload.py" in an extension folder and declaring a preload(parser) method, we can add extra command-line args for an extension.

* Save image/s before doing highres fix.

fix broken Restart Gradio button

option to use xattention optimizations when training
option to unload vae when training

Added "--clip-models-path" switch to avoid using default "~/.cache/clip" and enable to run under unprivileged user without homedir