message: add is_constant_initializer
diff: @@ -994,6 +994,9 @@ struct Graph final {
  }
  return initializers_.end();
  }
  + bool is_constant_initializer(const Value* value) const {
  + return value->node() == initializer_node_;
  + }
  ArrayRef<Value*> inputs() {
  return input_->outputs();
  }
  