Keras

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.

Keras 2.6.0 RC2 is a minor bug-fix release.

1. Fix TextVectorization layer with output_sequence_length on unknown input shapes.
2. Output int64 by default from Discretization layer.
3. Fix serialization of Hashing layer.
4. Add more explicit error message for instance type checking of optimizer.