PyTorch is a Python deep learning library that's currently gaining a lot of traction, because it's a lot easier to debug and prototype (compared to TensorFlow / Theano).
To install PyTorch on the Duckietbot you have to compile it from source, because there is no pro-compiled binary for ARMv7 / ARMhf available. This guid will walk you through the required steps.
First you need to install some additional packages. You might already have installed. If you do, that's not a problem.