code: def _numpy_kernel(self, amplitude, length_scale, x, y, df=None):
      if df is None:
          df = np.pi
      norm = np.sqrt(2 * df * np.sum((x - y) ** 2)) / length_scale
      return (
          amplitude**2
          * 2 ** (1 - df)
          / scipy_special.gamma(df)
          * norm**df
          * scipy_special.kv(df, norm)
      )
  
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 2 for calling the GeneralizedMaternTest obj's underlying member method code with example usage: obj._numpy_kernel(amplitude, length_scale, x, y, df) and returns: unknown