code: def get_stacked_pianoroll(self):
          
          multitrack = deepcopy(self)
          multitrack.pad_to_same()
          stacked = np.stack([track.pianoroll for track in multitrack.tracks], -1)
          return stacked
docs: Return a stacked multitrack pianoroll. The shape of the return array is
          (n_time_steps, 128, n_tracks).
  
          Returns
          -------
          stacked : np.ndarray, shape=(n_time_steps, 128, n_tracks)
              The stacked pianoroll.