New Problem: Diffusion related questions

[mavleo96]

Hi,

I want contribute some questions on diffusion models:

Problems:

1. DPPM - forward and backward
2. DDIM
3. Training a toy diffusion U-Net model
4. Classifier Free Guidance

Lemme know your thoughts!

[moe18]

that would be great! wanted to create some diffusion-based questions, just make sure to split them up into different PRs