docstr: Test of stacked parallel ccmbinators.
docstr_tok: ["Test","of","stacked","parallel","ccmbinators","."]
code: def test_stack_parallel(self):
      """Test of stacked parallel ccmbinators."""
      dataset1 = lambda: (i for i in range(10))
      dataset2 = lambda: (i for i in range(10, 20))
      dataset3 = lambda: (i for i in range(20, 30))
      parallel_lev0 = data.Parallel([dataset1, dataset2])
      parallel_lev1 = data.Parallel([parallel_lev0, dataset3])
      generator = parallel_lev1()
  
      self.assertEqual(next(generator), 0)
      self.assertEqual(next(generator), 20)
      self.assertEqual(next(generator), 10)
      self.assertEqual(next(generator), 21)
      self.assertEqual(next(generator), 1)
      self.assertEqual(next(generator), 22)
      self.assertEqual(next(generator), 11)
      self.assertEqual(next(generator), 23)
      self.assertEqual(next(generator), 2)
      self.assertEqual(next(generator), 24)
      self.assertEqual(next(generator), 12)
  
imports: import itertools
  import os
  from absl.testing import absltest
  from absl.testing import parameterized
  import numpy
  from trax import data
label_desc: Use image node_id 18 for calling the InputsTest obj's underlying member method code with example usage: obj.test_stack_parallel() without return types