'Returns the derivative of the binary cross entropy cost.
Args:
y (Tensor or OpTree): Output of previous layer or model
t (Tensor or OpTree): True targets corresponding to y
Returns:
OpTree: Returns the (mean) shortcut derivative of the binary entropy
cost function ``(y - t) / y.shape[1]``'

def bprop(self, y, t):

    return (self.scale * (y - t))
