message: Replace ValueError with assert since the `devices` argument is optional and change the error message to make it more descriptive.
diff: @@ -376,9 +376,8 @@ class MirroredExtended(distribute_lib.StrategyExtendedV1):
  super(MirroredExtended, self).__init__(container_strategy)
  if devices is None:
  devices = all_devices()
  - if not devices:
  - raise ValueError("Got an empty `devices` list. Please make sure the "
  - "`devices` you pass in is not empty.")
  + assert devices, ("Got an empty `devices` list and unable to recognize "
  + "any local devices.")
  self._cross_device_ops = cross_device_ops
  self._initialize_strategy(devices)
  