[FEATURE REQUEST] Add model type "Unet" to allow model download into the models/unet folder

[S.T. Tsao]

Currently for model download we have the choice of Checkpoint, LoRA, VAE, Embedding, and ControlNet. By choosing the right model type, the downloaded model is place in the right directory.

What is missing is the Unet model type, which will allow the placement of Unet model such as Stablity Cascade into the models/unet directory.