>PyTorch is almost (or even literally?) a fork of chainer
That's funny, I would have assumed PyTorch to be, like, the python version of Torch?
Th autograd library was inspired by Chainer's design and took a lot of concepts (but not code) directly from Chainer. The neural network API is a bit of a hybrid. It's built on top of the autograd library but the layer names, implementations, and some conventions were inherited from Torch 7's NN and cuNN libraries.
(EDIT: and the name "autograd" originates from HIPS autograd library, which I think predates Chainer)