DDPM Notebook merge request

[alihatamitajik]

Modifications after that have been done after the review session:

• 50 Epoch on Fashion MNIST -> no significant improvement on results
• Swiss-Roll data added for more comprehension
• doc string added to DDPM-related functions
• CVPR 2022 is mentioned in the resources directly
• some typos fixed

@behroozazarkhalili @mahsayazdani