They don't seem to explicitly say it, but it might be using the same core code given the structure of the framework and their mentioning that it's a mature codebase several years old. The license file also goes back to NYU before being taken over by Facebook, similar to Torch.

The core libraries are the same as in Lua torch, but the interface is redesigned and new.

They are sharing the code base using git-subtrees. So the C and CUDA parts of the codebase will be kept in sync. The modules written in lua or python will diverge.

I was wondering the same thing. There's even another repo that seems mildly popular called pytorch on Github: https://github.com/hughperkins/pytorch

They share the same underlying C and CUDA libraries. The Python and Lua modules are different. You can see both projects have pretty much the same contributors because they are sharing the code base using git-subtree.