I don't think this is what they are actually using, but path tracing can be reduced to a really huge sparse sampling problem. This means that given any predictive model of how much paths through the scene will add light to a pixel, you can make that model unbiased by using https://en.wikipedia.org/wiki/Importance_sampling

So a model trained to take the samples you have so far and predict paths that will also connect eye to light sources could hugely speed up rendering. I don't know of any software that does this, so free idea if someone wants to try it.