code: def derivatives(self, x, y, amp, sigma_x, sigma_y, center_x=0, center_y=0):
          
          f_ = self.function(x, y, amp, sigma_x, sigma_y, center_x, center_y)
          return f_ * (center_x-x)/sigma_x**2, f_ * (center_y-y)/sigma_y**2
docs: returns df/dx and df/dy of the function