docs: def evaluate(self, flux, xo, yo, a, b, c):
          """
          Evaluate the Gaussian model
  
          Parameters
          ----------
          flux : tf.Variable
          xo, yo : tf.Variable, tf.Variable
              Center coordiantes of the Gaussian.
          a, b, c : tf.Variable, tf.Variable
              Parameters that control the rotation angle
              and the stretch along the major axis of the Gaussian,
              such that the matrix M = [a b ; b c] is positive-definite.
  
          References
          ----------
          https://en.wikipedia.org/wiki/Gaussian_function#Two-dimensional_Gaussian_function
          """
          dx = self.x - xo
          dy = self.y - yo
          psf = tf.exp(-(a * dx ** 2 + 2 * b * dx * dy + c * dy ** 2))
          psf_sum = tf.reduce_sum(psf)
          return flux * psf / psf_sum