docstr: Basic test of the parallel ccmbinator.
docstr_tok: ["Basic","test","of","the","parallel","ccmbinator","."]
code: def test_parallel(self):
      """Basic test of the parallel ccmbinator."""
      dataset1 = lambda: (i for i in range(10))
      dataset2 = lambda: (i for i in range(10, 20))
      parallel = data.Parallel([dataset1, dataset2])
      generator = parallel()
  
      self.assertEqual(next(generator), 0)
      self.assertEqual(next(generator), 10)
      self.assertEqual(next(generator), 1)
      self.assertEqual(next(generator), 11)
      self.assertEqual(next(generator), 2)
      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 10 for calling the InputsTest obj's underlying member method code with example usage: obj.test_parallel() without return types