code: def get_active_length(self):
          
          active_length = 0
          for track in self.tracks:
              now_length = track.get_active_length()
              if active_length < track.get_active_length():
                  active_length = now_length
          return active_length
docs: Return the maximum active length (i.e., without trailing silence) among
          the pianorolls of all tracks. The unit is time step.
  
          Returns
          -------
          active_length : int
              The maximum active length (i.e., without trailing silence) among the
              pianorolls of all tracks. The unit is time step.