variables.

…` argument.

…ave GPU kernels.

… to their saving and restoring mechanism.

* Move performance heuristics in Distribution Strategy level. We will not expose knobs for users to control;
* Emphasize that embedding support in v2 will all be via `Embedding` layer. Users can use `tf.compat.v1` to handle embedding by themselves;
* Mention that default `partition_strategy` in v1 `embedding_lookup` is "mod", which will possibly break users's model when they update to TF 2.0;
* We want to prioritize shuffling embedding after 2.0 release;
* We have plans to serialize and deserialize `Embedding` layer and Distribution Strategies to allow loading a saved model to a different number of partitions.