BoostedTreesRegressor.
*   Add 3rd generation pipeline config for Cloud TPUs which improves performance
    and usability.
*   `tf.contrib.bayesflow` is moving out to it's own repo.
*   Added `tf.contrib.{proto,rpc}` to allow generic proto parsing and RPC
    communication<sup>[1](#rpc-issue)</sup>.

## Bug Fixes and Other Changes

*   `tf.data`:
    *   Add `tf.contrib.data.prefetch_to_device`, which enables prefetching