message: function minor optimization
diff: @@ -1691,7 +1691,7 @@ class ConcreteFunction(object):
  default_graph.mark_as_unsaveable(self._func_graph.saving_errors)
  if (tape.could_possibly_record() or
  - hasattr(ops.get_default_graph(), "watch_variable")):
  + hasattr(default_graph, "watch_variable")):
  for v in self._func_graph.variables:
  resource_variable_ops.variable_accessed(v)
  @@ -1754,7 +1754,7 @@ class ConcreteFunction(object):
  ctx, args_with_tangents,
  cancellation_manager=cancellation_manager)
  else:
  - with ops.get_default_graph()._override_gradient_function( # pylint: disable=protected-access
  + with default_graph._override_gradient_function( # pylint: disable=protected-access
  {"PartitionedCall": self._get_gradient_function(),
  "StatefulPartitionedCall": self._get_gradient_function()}):
  flat_outputs = forward_function.call(ctx, args_with_tangents)
  