GradientDef

 GradientDef defines the gradient function of a function defined in
 a function library.
 A gradient function g (specified by gradient_func) for a function f
 (specified by function_name) must follow the following:
 The function 'f' must be a numerical function which takes N inputs
 and produces M outputs. Its gradient function 'g', which is a
 function taking N + M inputs and produces N outputs.
 I.e. if we have
    (y1, y2, ..., y_M) = f(x1, x2, ..., x_N),
 then, g is
    (dL/dx1, dL/dx2, ..., dL/dx_N) = g(x1, x2, ..., x_N,
                                      dL/dy1, dL/dy2, ..., dL/dy_M),
 where L is a scalar-value function of (x1, x2, ..., xN) (e.g., the
 loss function). dL/dx_i is the partial derivative of L with respect
 to x_i.