Question about the update method

[zhzou2020]

For u, the gradient should be (1-sigmoid(uv))*v when the label is 1, (sigmoid(-uv) - 1)*v when the label is 0.

But in the update method, the gradient becomes (1-sigmoid(uv))*v when the label is 1, -sigmoid(uv)*v when the label is 0.

Is there any error in my calculation? Or this is a bug of the code?