code: def testNegativeSigmaFails(self):
      with self.assertRaisesOpError(
          "Argument `scale` must be positive."
      ):
          dist = normal.Normal(
              loc=[1.0], scale=[-5.0], validate_args=True, name="G"
          )
          self.evaluate(dist.mean())
  
imports: import math
  import numpy
  from scipy import stats
  import tensorflow.compat.v1
  import tensorflow.compat.v2
  from tensorflow_probability.python.distributions import kullback_leibler
  from tensorflow_probability.python.distributions import normal
  from tensorflow_probability.python.internal import test_util
  from tensorflow_probability.python.math import gradient
label_desc: Use image node_id 26 for calling the NormalTest obj's underlying member method code with example usage: obj.testNegativeSigmaFails() without return types