torch.autograd.Function throw error could not compute gradients

autograd
1

recently implement the code from the paper shake-shake(https://pdfs.semanticscholar.org/22aa/426aeffb77339646cc03da8e94de22396efc.pdf)

class ShakeNoiseFunction(torch.autograd.Function):
    def forward(self, x1,x2):
        weight1 = torch.rand(1)
        weight2 = 1 - weight1
        x1 = x1 * weight1.numpy().tolist()[0]
        x2 = x2 * weight2.numpy().tolist()[0]
        return x1 + x2
    def backward(self, x):
        g = x[0]
        weight1 = torch.rand(1)
        weight2 = 1 - weight1
        g1 = g * weight1.numpy().tolist()[0]
        g2 = g * weight2.numpy().tolist()[0]
        return g1,g2

but throw the error “could not compute gradients for some functions”