a = Value(2.0)
b = Value(3.0)
c = a * b       # c = 6.0
L = c + a       # L = 8.0
L.backward()
print(a.grad)   # 4.0 (dL/da = b + 1 = 3 + 1, via both paths)
print(b.grad)   # 2.0 (dL/db = a = 2)