Improve U-Net and Transformer implementations

[cabralpinto]

As the documentation states, the current implementations are neither the most effective or efficient. The U-Net implementation was adapted from the The Annotated Diffusion Model and the Transformer implementation was adapted from Peebles & Xie (2022) (adaptive layer norm block). Although these produce good enough results, ideally the library would provide the best implementations out there for general use.

From what I've read, I think a good choice for the U-Net implementation would be the one used in Imagen for the Text-to-Image model, but there may well be other more recent architectures that would be a better fit. For the Transformer I'm really not sure right now. Any input on this would be greatly appreciated.