message: [tf.data] Properly export `tf.contrib.data.group_by_reducer()`
diff: @@ -33,6 +33,7 @@ See the @{$datasets$Importing Data} Programmer's Guide for an overview.
  @@choose_from_datasets
  @@dense_to_sparse_batch
  @@enumerate_dataset
  +@@group_by_reducer
  @@group_by_window
  @@ignore_errors
  @@make_batched_features_dataset
  @@ -71,6 +72,7 @@ from tensorflow.contrib.data.python.ops.enumerate_ops import enumerate_dataset
  from tensorflow.contrib.data.python.ops.error_ops import ignore_errors
  from tensorflow.contrib.data.python.ops.get_single_element import get_single_element
  from tensorflow.contrib.data.python.ops.grouping import bucket_by_sequence_length
  +from tensorflow.contrib.data.python.ops.grouping import group_by_reducer
  from tensorflow.contrib.data.python.ops.grouping import group_by_window
  from tensorflow.contrib.data.python.ops.interleave_ops import choose_from_datasets
  from tensorflow.contrib.data.python.ops.interleave_ops import parallel_interleave
  