Adds an entropy-based compression algorithm for compressing client updates (as detailed in https://arxiv.org/abs/2201.02664) to tff.learning, as well as an aggregator for encoding client updates using the run_length_gamma_encode op hosted in TFC (https://www.tensorflow.org/api_docs/python/tfc/ops/run_length_gamma_encode).

[copybara-service[bot]]

Adds an entropy-based compression algorithm for compressing client updates (as detailed in https://arxiv.org/abs/2201.02664) to tff.learning, as well as an aggregator for encoding client updates using the run_length_gamma_encode op hosted in TFC (https://www.tensorflow.org/api_docs/python/tfc/ops/run_length_gamma_encode).