message: Add doc for extended._retrace_functions_for_each_device.
diff: @@ -619,6 +619,11 @@ class StrategyBase(object):
  if not hasattr(extended, "_retrace_functions_for_each_device"):
  # pylint: disable=protected-access
  + # `extended._retrace_functions_for_each_device` dictates
  + # 1) whether all the ops created inside function will have devices
  + # inherited from outer stack, and
  + # 2) whether the same function will be retraced when it is called on
  + # different devices.
  try:
  extended._retrace_functions_for_each_device = (
  len(extended.worker_devices) > 1)
  