*   Fix the issue that `tf.distribute.MirroredStrategy` cannot be used together
    with `tf.distribute.experimental.MultiWorkerMirroredStrategy`.
*   Add a `tf.distribute.cluster_resolver.TPUClusterResolver.connect` API to
    simplify TPU initialization.
*   Add `tf.distribute.Strategy.gather` and
    `tf.distribute.ReplicaContext.all_gather` methods to gather and concatenate
    `tf.distribute.DistributedValues` across workers and devices.