Method is_open

oneflow/extension/stack/stacktrace.h:3612–3612

    if (!_file || !is_open()) { _file.reset(new std::ifstream(path.c_str())); }
  }
  bool is_open() const { return _file->is_open(); }

  lines_t& get_lines(unsigned line_start, unsigned line_count, lines_t& lines) {
    using namespace std;