TensorFlow Federated 0.81.0

Release 0.81.0

Added

• A helper function to get a vector of strings for the elements of a tensor in
  order to aid in formatting.
• A field string_val to the tensor proto to allow representing string
  values explicitly.

Changed

• The format of the release notes (i.e., RELEASE.md) to be based on
  https://keepachangelog.com/en/1.1.0/.
• Moved constraint on linfinity_bound from DPGroupingFederatedSumFactory
  to DPGroupByFactory, because closed-domain histogram algorithm will use
  DPGroupingFederatedSum but not demand a positive linfinity_bound.

Removed

• The dependency on semantic-version.
• The tff.async_utils package, use asyncio instead.