message: Add 'device' property to TPUMirroredVariable, so tf.train.init_from_checkpoint can be supported.
diff: @@ -571,6 +571,10 @@ class TPUMirroredVariable(checkpointable.CheckpointableBase):
  ValueError("Device %s not found in %s (current device %s)" %
  (device, self._index.keys(), device_util.current())), e)
  + @property
  + def device(self):
  + return self._get().device
  +
  # The arguments to update() are automatically unwrapped so the update()
  # function would normally see regular variables, not MirroredVariables.
  # However, the update function can still operate on wrapped MirroredVariables
  