support v prediction models
Add support for an arg that allows support of SD 2.x models with v prediction
@lllyasviel in case you were hesitant to merge just in case it didn't work, I did the circles test on WD 1.5, which uses v prediction and training went just fine. The following images are from step 3300 @ 4 batch size

Also just wanted to say really nice work that you guys are doing. I must admit I was pretty surprised to see Waifu Diffusion in the research paper, pretty cool to see something I worked on actually getting used for research lol
on my extended training runs vs non v prediction models, something is definitely not working right. Closing this PR until I can figure out what it is and fix it.