*   Other:
    *   tf.py_func now reports the full stack trace if an exception occurs.
    *   Integrate `TPUClusterResolver` with GKE's integration for Cloud TPUs.
    *   Add a library for statistical testing of samplers.
    *   Add Helpers to stream data from the GCE VM to a Cloud TPU.
    *   Integrate ClusterResolvers with TPUEstimator.
    *   Unify metropolis_hastings interface with HMC kernel.