code: def testCallFnOneArg(self, arg):
      def fn(arg):
          return arg
  
      self.assertEqual(
          tf.nest.flatten(arg),
          tf.nest.flatten(nest_util.call_fn(fn, arg)),
      )
  
imports: import collections
  from absl.testing import parameterized
  import numpy
  import tensorflow.compat.v2
  from tensorflow_probability.python.distributions import deterministic
  from tensorflow_probability.python.distributions import half_cauchy
  from tensorflow_probability.python.distributions import joint_distribution_auto_batched
  from tensorflow_probability.python.distributions import normal
  from tensorflow_probability.python.internal import nest_util
  from tensorflow_probability.python.internal import test_util
  from tensorflow.python.util import nest
label_desc: Use image node_id 6 for calling the NestUtilTest obj's underlying member method code with example usage: obj.testCallFnOneArg(arg) and returns: arg