message: Fix flakiness in map_dataset_op_test.
diff: @@ -275,7 +275,7 @@ py_test(
  py_test(
  name = "map_dataset_op_test",
  - size = "small",
  + size = "medium",
  srcs = ["map_dataset_op_test.py"],
  srcs_version = "PY2AND3",
  deps = [
  