message: [tf.data] Avoid calling `Iterator.get_next()` in each iteration of graph mode.
diff: @@ -45,8 +45,8 @@ class DatasetTestBase(test.TestCase):
  ```python
  # In both graph and eager modes
  dataset = ...
  - nxt = self.getNext(dataset)
  - result = self.evaluate(nxt())
  + get_next = self.getNext(dataset)
  + result = self.evaluate(get_next())
  ```
  Args:
  @@ -66,7 +66,8 @@ class DatasetTestBase(test.TestCase):
  self.evaluate(iterator.initializer)
  else:
  iterator = dataset.make_one_shot_iterator()
  - return iterator.get_next
  + get_next = iterator.get_next()
  + return lambda: get_next
  def _compareOutputToExpected(self, result_values, expected_values,
  assert_items_equal):
  