docstr: Check that data.Serial works inside another data.Serial.
docstr_tok: ["Check","that","data.Serial","works","inside","another","data.Serial","."]
code: def test_parallel_with_weights_three_datasets(self):
      """Check that data.Serial works inside another data.Serial."""
      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 = data.Parallel(
          [dataset1, dataset2, dataset3], counters=(2, 1, 3)
      )
      generator = parallel()
  
      self.assertEqual(next(generator), 0)  # (1,0,0)
      self.assertEqual(next(generator), 10)  # (1,1,0)
      self.assertEqual(next(generator), 20)  # (1,1,1)
      self.assertEqual(next(generator), 1)  # (2,1,1)
      self.assertEqual(next(generator), 21)  # (2,1,2)
      self.assertEqual(next(generator), 22)  # (2,1,3)
      self.assertEqual(next(generator), 2)  # (1,0,0)
      self.assertEqual(next(generator), 11)  # (1,1,0)
      self.assertEqual(next(generator), 23)  # (1,1,1)
      self.assertEqual(next(generator), 3)  # (2,1,1)
      self.assertEqual(next(generator), 24)  # (2,1,2)
      self.assertEqual(next(generator), 25)  # (2,1,3)
      self.assertEqual(next(generator), 4)
  
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 17 for calling the InputsTest obj's underlying member method code with example usage: obj.test_parallel_with_weights_three_datasets() without return types