message: Update docs of tf.distribute.StrategyExtended
diff: @@ -1540,6 +1540,7 @@ class StrategyExtendedV2(object):
  @property
  def experimental_require_static_shapes(self):
  + """Returns `True` if static shape is required; `False` otherwise."""
  return self._require_static_shapes
  @property
  @@ -1570,6 +1571,8 @@ class StrategyExtendedV2(object):
  Args:
  var_list: The list of variables being optimized, needed with the
  default `tf.distribute.Strategy`.
  + Returns:
  + A sequence of devices for non-slot variables.
  """
  raise NotImplementedError("must be implemented in descendants")
  