code: def testGradsNoneDfParameter(self):
      x = tf.constant([5.0], dtype=np.float32)
      y = tf.constant([-2.0], dtype=np.float32)
  
      def _apply(df):
          k = matern.GeneralizedMatern(
              df=df, amplitude=1.0, length_scale=1.0, feature_ndims=1
          )
          return k.apply(x, y)
  
      df = tf.constant(2.5, dtype=np.float32)
  
      self.assertIsNone(gradient.value_and_gradient(_apply, df)[1])
  
imports: from absl.testing import parameterized
  import numpy
  import scipy.special
  import tensorflow.compat.v2
  from tensorflow_probability.python.internal import test_util
  from tensorflow_probability.python.math import gradient
  from tensorflow_probability.python.math.psd_kernels import matern
label_desc: Use image node_id 9 for calling the GeneralizedMaternTest obj's underlying member method code with example usage: obj.testGradsNoneDfParameter() and returns: k