Keras

Please see the release history at https://github.com/tensorflow/tensorflow/releases/tag/v2.7.0 for more details.

What's Changed
* Fix tf_idf output mode for lookup layers by mattdangerw in https://github.com/keras-team/keras/pull/15492
* Disable the failing tests due to numpy 1.20 change by qlzh727 in https://github.com/keras-team/keras/pull/15552


**Full Changelog**: https://github.com/keras-team/keras/compare/v2.7.0-rc1...v2.7.0-rc2

Cherrypicked the documentation update for functional model slicing.

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.

Keras Release 2.6.0 RC3 fix a security issue for loading keras models via yaml, which could allow arbitrary code execution.