code: def _cache_dataset(dataset, prefix):
      
      if not os.path.exists(_constants.CACHE_PATH):
          os.makedirs(_constants.CACHE_PATH)
      src_data = np.concatenate([e[0] for e in dataset])
      tgt_data = np.concatenate([e[1] for e in dataset])
      src_cumlen = np.cumsum([0]+[len(e[0]) for e in dataset])
      tgt_cumlen = np.cumsum([0]+[len(e[1]) for e in dataset])
      np.savez(os.path.join(_constants.CACHE_PATH, prefix + '.npz'),
               src_data=src_data, tgt_data=tgt_data,
               src_cumlen=src_cumlen, tgt_cumlen=tgt_cumlen)
docs: Cache the processed npy dataset the dataset into a npz
  
      Parameters
      ----------
      dataset : SimpleDataset
      file_path : str