code: def get_nested_backend(self):
      return MarsDistributedBackend(session=self.session), -1
  
file_path: mars/mars/learn/contrib/joblib/backend.py
imports: import concurrent.futures
  from ....None import remote
  from ....session import get_default_session, new_session
label_desc: Use image node_id 2 for calling the MarsDistributedBackend obj's underlying member method code with example usage: obj.get_nested_backend() and returns: MarsDistributedBackend,  unknown