bool IsPersistent() const { return persistent_; }

 private:
  TensorTape tensor_tape_;
  OpTape<BackwardFunction, TapeTensor> op_tape_;
  int64_t next_op_id_{0};

  // Map from tensor id to number of remaining usages (i.e. how many entries in
  // the tape refer to it); to aid in tape garbage collection.
  std::unordered_map<int64_t, int64_t> tensor_usage_;

*   Improved the performance of CPU assignment for strings.
*   Speed up matrix * vector multiplication and matrix * matrix with unknown
    shapes.
*   C API: Graph imports now support input remapping, control dependencies, and
    returning imported nodes (see `TF_GraphImportGraphDefWithReturnOutputs()`)
*   Multiple C++ API updates.
*   Multiple TensorBoard updates including: