Code sample in this section: https://github.com/Devinterview-io/pytorch-interview-questions?tab=readme-ov-file#code-example-autograd-in-action

x.grad is the partial derivative of w with respect to x, which is $\frac{\partial{w}}{\partial{z}}$ $\frac{\partial{z}}{\partial{x}}$
$\frac{\partial{w}}{\partial{z}}$ = 2ドルx$
$\frac{\partial{z}}{\partial{x}}$ = $y$
Thus $\frac{\partial{w}}{\partial{z}}$ $\frac{\partial{z}}{\partial{x}}$ = 2ドルzy$. $y$ was initialized to 4, and $z$ = $xy$ = 12. So 2ドルzy$ = 2 times 12 times 4 = 96.

Likewise you can work out the value of y.grad using the chain rule as well, and it's 72.