// Accumulate will forward op executions to the tape while the backward
  // function is running; this effectively adds the backward tape to the active
  // set (but does not require complicated callbacks to the language bindings).
  Status ForwardpropFromTape(
      const string& op_type, const std::vector<TapeTensor>& output_tensors,
      const std::function<BackwardFunction*()>& backward_function_getter,