code: def testNegativeSigmaFails(self):
      with self.assertRaisesError("Argument `scale` must be positive."):
          halfnorm = half_normal.HalfNormal(
              scale=[-5.0], validate_args=True, name="G"
          )
          self.evaluate(halfnorm.mean())
  
imports: from absl.testing import parameterized
  import numpy
  from scipy import stats
  import tensorflow.compat.v1
  import tensorflow.compat.v2
  from tensorflow_probability.python.distributions import half_normal
  from tensorflow_probability.python.distributions import kullback_leibler
  from tensorflow_probability.python.internal import test_util
  from tensorflow_probability.python.math import gradient
label_desc: Use image node_id 23 for calling the HalfNormalTest obj's underlying member method code with example usage: obj.testNegativeSigmaFails() without return types