At the bottom of chapter 15 https://github.com/fastai/fastbook/blob/5cf040c55c7da95167595ffa815499d37eb266f2/15_arch_details.ipynb , Further research bullet #4 says
Read the online chapter about generative image models, and create your own colorizer, super-resolution model, or style transfer model.
Does this online chapter exist yet or is it something to be released in the future?