I do want to clarify that PyTorch was not a fork of Chainer in the literal sense. The frontend design was inspired by Chainer, but the devil is in the details – it was a full implementation of a frontend and engine with the backend connected to Torch-7’s C libraries.
Here’s more info: PyTorch Tutorial for Deep Learning Researchers 2
As of today, the architecture diagram looks roughly like this:

pt_execution.png506×635 39.3 KB

Below “Execute” stands the ATen library.
We have had great meetings and chats with the Chainer team at various times, especially at conferences, but also in-person at their offices. Many positive things have come out, but not the ability to reconcile communities (unfortunately).