Partial update of tf.keras to the Keras 2.1.6 API.

Partial update of tf.keras to the Keras 2.1.6 API.

This covers the following features and associated unit tests:
- multi-output layer where `compute_output_mask` returns `None`.
- saving to, and loading from, an existing hdf5 file.
- `verbose` argument (1/0) in `evaluate_generator`.
- stateful metrics with generator methods.
- `data_format` argument in `Flatten`.
- `constants` argument in Bidirectional's `__call__`.

PiperOrigin-RevId: 196198134