code: def _numpy_kernel(self, amplitude, length_scale, x, y):
      norm = np.sqrt(3.0 * np.sum((x - y) ** 2)) / length_scale
      return amplitude**2 * (1 + norm) * np.exp(-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 1 for calling the MaternThreeHalvesTest obj's underlying member method code with example usage: obj._numpy_kernel(amplitude, length_scale, x, y) and returns: unknown