Keras Release 2.6.0 RC0

Keras 2.6.0 is the first release of TensorFlow implementation of Keras in the present repo.

The code under tensorflow/python/keras is considered legacy and will be removed in future releases (tf 2.7 or later). For any user who import tensorflow.python.keras, please update your code to public tf.keras instead.

The API endpoints for tf.keras stay unchanged, but are now backed by the keras PIP package. All Keras-related PRs and issues should now be directed to the GitHub repository keras-team/keras.

For the detailed release notes about tf.keras behavior changes, please take a look for tensorflow release notes.