Simon Wang’s response there is a very good “quick and high-level description.”
Follow Peter Goldsborough’s tutorial 43 for a hands-on introduction to interfacing between ATen and Python. The way the PyTorch source interfaces isn’t exactly the same, but the tutorial will get you acquainted with ATen.
If you want to dig further, this blog post 24 has a good tour of the internals of ATen.
Do be aware that once you get to the T*C level, you’re up against daily changes (see goldsborough’s first reply on this thread 18).