`tf.contrib.data.choose_from_datasets()` make it easier to sample or
        deterministically choose elements from multiple datasets.
    *   `tf.contrib.data.make_csv_dataset()` now supports line breaks in quoted
        strings, and two infrequently used arguments removed.
    *   (C++) `DatasetBase::DebugString()` is now `const`.
    *   (C++) `DatasetBase::MakeIterator()` has been renamed to