code: def upstream_structure(self):
          
          structure_dict = {'edges': set(),
                            'nodes': set()}
          structure_dict = self._build_structure_dict(structure_dict)
          return structure_dict
docs: Build dictionary with entire upstream pipeline structure
          (with regard to the current Step).
  
          Returns:
              dict: dictionary describing the upstream pipeline structure. It has two keys:
              ``'edges'`` and ``'nodes'``, where:
  
              - value of ``'edges'`` is set of tuples ``(input_step.name, self.name)``
              - value of ``'nodes'`` is set of all step names upstream to this Step