template <typename BackwardFunction, typename TapeTensor>
struct OpTapeEntry {
  string op_type;
  std::vector<TapeTensor> output_tensor_info;
  std::vector<int64_t> input_tensor_id;

  // TODO(apassos) consider narrowing down this interface.
  BackwardFunction* backward_function;

  // Should be called before deleting the backward function. TODO(apassos) use
  // unique_ptrs to ensure this happens.