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

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

Changes included are:
- Fix `batch_dot` when `axes=None`
- Add axis=-1 as an argument to keras.backend.softmax
- Fix ctc_batch_cost() error when batch_size = 1
- Print previous best in ModelCheckpoint callback
- Fix ReduceLROnPlateau callback
- Extend RemoteMonitor to send data as application/json
- Fix default dilation rate value in 2D separable conv.
- Fix for MobileNet model with undefined shape
- Disable require_flatten in nasnet & Add an error message for undefined shape.
- Improve tests by designating dtype of sample data
- Multi_gpu_model supporting legacy/fullCPU/fullGPU

PiperOrigin-RevId: 196615376