Keras is probably the simplest way to get started. You declare your model using the provided layers, and then you train it fairly simply. Implementing custom techniques is not so easy.
PyTorch is great for running experiments, and it makes implementing custom techniques fairly easy.
Tensorflow is popular but harder to learn than PyTorch.
Contributing to PyTorch probably isn’t the best way of getting started. Trying to help out on the forum is better, but the best way is to find a few datasets and practice building models. https://medium.com/ai³-theory-practice-business/best-method-to-learn-essential-machine-learning-skills-fast-533e30f3023d 3