code: def save(self, vleaf, fpath, cleanup=False, format=None):
          
          graph = self.create_graphviz_digraph(vleaf, format=format)
          graph.render(fpath, cleanup=cleanup)
docs: Save the graph to a given file path.
  
          Args:
            vleaf (`nnabla.Variable`): End variable. All variables and functions which can be traversed from this variable are shown in the reuslt.
            fpath (`str`): The file path used to save. 
            cleanup (`bool`): Clean up the source file after rendering. Default is False.
            format (str):
                Force overwrite ``format`` (``'pdf', 'png', ...)``) configuration.