code: def test_nested_reducers(self):
      fake_kernel = test_fixtures.TestTransitionKernel()
      fake_reducers = [
          [
              test_fixtures.NaiveMeanReducer(),
              covariance_reducer.CovarianceReducer(),
          ],
          [test_fixtures.NaiveMeanReducer()],
      ]
      reduction_rslt, last_sample, kr = sample_fold(
          num_steps=3,
          current_state=0.0,
          kernel=fake_kernel,
          reducer=fake_reducers,
          seed=test_util.test_seed(),
      )
      reduction_rslt, last_sample, kernel_results = self.evaluate(
          [reduction_rslt, last_sample, kr]
      )
      self.assertEqual(2, len(reduction_rslt))
      self.assertEqual(2, len(reduction_rslt[0]))
      self.assertEqual(1, len(reduction_rslt[1]))
  
      self.assertEqual(2, reduction_rslt[0][0])
      self.assertNear(2 / 3, reduction_rslt[0][1], err=1e-6)
      self.assertEqual(3, last_sample)
      self.assertEqual(3, kernel_results.counter_1)
      self.assertEqual(6, kernel_results.counter_2)
  
imports: import collections
  import warnings
  from absl.testing import parameterized
  import numpy
  import tensorflow.compat.v2
  from tensorflow_probability.python.distributions import normal
  from tensorflow_probability.python.experimental.mcmc import covariance_reducer
  from tensorflow_probability.python.experimental.mcmc import expectations_reducer
  from tensorflow_probability.python.experimental.mcmc.internal import test_fixtures
  from tensorflow_probability.python.experimental.mcmc.sample_fold import sample_chain_with_burnin
  from tensorflow_probability.python.experimental.mcmc.sample_fold import sample_fold
  from tensorflow_probability.python.internal import samplers
  from tensorflow_probability.python.internal import tensorshape_util
  from tensorflow_probability.python.internal import test_util
  from tensorflow_probability.python.mcmc import hmc
label_desc: Use image node_id 5 for calling the SampleFoldTest obj's underlying member method code with example usage: obj.test_nested_reducers() without return types