code: def testHalfCauchyStddev(self):
      batch_size = 3
      loc = self._create_placeholder_with_default(
          [0.0] * batch_size, name="loc"
      )
      scale = self._create_placeholder_with_default(1.0, name="scale")
  
      dist = half_cauchy.HalfCauchy(
          loc, scale, allow_nan_stats=False, validate_args=True
      )
      with self.assertRaisesRegexp(ValueError, "is undefined"):
          self.evaluate(dist.stddev())
  
      dist = half_cauchy.HalfCauchy(
          loc, scale, allow_nan_stats=True, validate_args=True
      )
      self.assertAllNan(dist.stddev())
  
imports: import numpy
  from scipy import stats
  import tensorflow.compat.v1
  import tensorflow.compat.v2
  from tensorflow_probability.python.distributions import half_cauchy
  from tensorflow_probability.python.internal import test_util
  from tensorflow_probability.python.math import gradient
label_desc: Use image node_id 19 for calling the _HalfCauchyTest obj's underlying member method code with example usage: obj.testHalfCauchyStddev() without return types