w1 = torch.tensor(-2.0, requires_grad=True)   # the only change
b1 = torch.tensor(0.0,  requires_grad=True)
w2 = torch.tensor(3.0,  requires_grad=True)
b2 = torch.tensor(1.0,  requires_grad=True)
x, y = torch.tensor(1.0), torch.tensor(2.0)

h1 = w1 * x + b1
a1 = torch.relu(h1)
h2 = w2 * a1 + b2
loss = (h2 - y) ** 2
loss.backward()

print(w1.grad.item(), b1.grad.item())   # 0.0 0.0
print(w2.grad.item(), b2.grad.item())   # 0.0 -2.0