TensorFlow Federated 0.29.0

Release 0.29.0

Major Features and Improvements

• Update the MemoryReleaseManager to save type_signature when releasing
  values.
• Add a type_signature parameter to the ReleaseManager.release method.
• Unify retryability logic between TFF-C++ and TFF-Python.
• Update the TFF contributions and collaboration links to point to the Discord
  server.

Breaking Changes

• Move Python executor stacks file to python_executor_stacks.py in
  executor_stacks directory.

Bug Fixes

• Ensure that dataset deserialization will return ragged and sparse tensors,
  as needed according to the TFF type of the dataset.
• Make metric_finalizers use metric constructors if available.