Added

• TopK layers now come with a query_with_exclusions method, allowing
  certain candidates to be excluded from top-k retrieval.
• TPUEmbedding Keras layer for accelerating embedding lookups for large
  tables with TPU.

Changed

• factorized_top_k.Streaming layer now accepts a query model, like other
  factorized_top_k layers.

• Updated ScaNN to 1.2.0, which requires TensorFlow 2.4.x. When not using
  ScaNN, any TF >= 2.3 is still supported.