Add JiT: Just image Transformer (JiT) for Pixel-space Diffusion

Pixel-level diffusion seems to be catching up and I think `diffusers` could have a nice role to play in accelerating its adoption and research.

Code and checkpoints can be found here: https://github.com/LTH14/JiT.git.

Even though the released checkpoints were obtained with ImageNet, I believe we can support this like we supported the original DiT when it came out.

Cc: @LTH14 (first author), @kashif