There is a thread about implementing fast.ai in pure Pytorch: Plain PyTorch implementation of fast.ai notebooks
To learn Pytorch you can start by the Udacity course here: https://www.udacity.com/course/deep-learning-pytorch--ud188
It is a hard work reimplementing fast.ai but I think you will learn a lot when you finish. I have done one about Tabular module (v 0.7) Reverse Tabular Module in fast.ai v1 . with other module like Vision or Text it would be much harder
Hope that helps,