Major Features and Improvements
* DTensor TF executor is now integrated with the default TFF C++ worker.
* Added federated program documentation and guidelines.
* Removed the `pytype` dependency from TFF.
* `tff.learning.algorithms.build_fed_recon_eval` now supports TFF optimizers.
Breaking Changes
* Updated `tff.types.deserialize_type` to not accept/return `None`.
* Removed the `tff.framework.ComputationBuildingBlock.is_foo` methods.
* Renamed `tff.learning.algorithms.build_personalization_eval` to
`tff.learning.algorithms.build_personalization_eval_computation`
* `tff.learning.models.ReconstructionModel.from_keras_model` will now check
that global and local variables are disjoint, raise ValueError if they are
not.
Bug Fixes
* Fixed `tff.learning.models.ReconstructionModel.has_only_global_variables`
(it was returning incorrect value).