Images as Tensors
Before any model sees a pixel, an image is a grid of numbers — and the conventions for arranging and scaling those numbers disagree between nearly every library you'll touch. Almost every mysterious vision bug traces back to one of these conventions being silently wrong: a colour channel swapped, a value range mismatched, an axis order flipped.
PyTorch Tensors and Autograd
Every gradient in the previous eleven pages was derived and coded by hand. From here on, a framework does that work — but only because it implements exactly the mechanism Backpropagation already described: recording a computational graph as operations run, then walking it backward. Autograd is the manual backward pass, automated and generalised to arbitrary graphs.