message: Expose tpu_host_placement_function().
diff: @@ -154,6 +154,20 @@ class TPUContext(object):
  # as far as model is replicated to all cores in the system.
  return self._internal_ctx.device_for_replica(replica_id)
  + @property
  + def tpu_host_placement_function(self):
  + """Returns the TPU host place function.
  +
  + The place function takes host_id as the input and returns the TF device
  + for the correspoding host.
  + """
  +
  + def _placement_function(host_id):
  + """Return the host device given host_id."""
  + return self._internal_ctx.tpu_host_placement_function(host_id=host_id)
  +
  + return _placement_function
  +
  class _InternalTPUContext(object):
  """A context holds immutable states of TPU computation.
  