When you are trying to learn PyTorch, I would suggest to pick an interesting (and personal) project you could spend some time on. E.g. if you are interested in photography and would like to experiment with some style transfer approaches, this would be a great way to learn more about GANs and other architectures. You would learn the framework just by working on the project. 
On the other hand, if you would like to contribute to PyTorch, I would recommend to have a look at the usability / simple-fixes or misc category here 1.
Also, the good first issue 1 label is useful to check for some starter PRs. The Contribution Guide 1 is a good way to get started.
Generally, I would say that a lot of new users would have some trouble with language models, i.e. how the shapes are used in RNNs, where and when to detach() the activations etc.