docstr: Returns a list of sub flows used in the combined_workflow. Needs to
  be implemented in every new combined_workflow.
docstr_tok: ["Returns","a","list","of","sub","flows","used","in","the","combined_workflow",".","Needs","to","be","implemented","in","every","new","combined_workflow","."]
code: def _get_sub_flows(self):
      """Returns a list of sub flows used in the combined_workflow. Needs to
      be implemented in every new combined_workflow.
      """
      raise AttributeError(
          "Error: _get_sub_flows() has to be defined for {}".format(
              self.__class__
          )
      )
  
file_path: dipy/dipy/workflows/combined_workflow.py
imports: from dipy.workflows.workflow import Workflow
label_desc: Use image node_id 3 for calling the CombinedWorkflow obj's underlying member method code with example usage: obj._get_sub_flows() without return types