docs: def savez_two_column(matrix, row_offset, file_name, append=False):
    """Savez_compressed obj to file_name."""
    logging.info("Saving obj to file in two column .npz format %s.", file_name)
    tc = []
    for u, items in enumerate(matrix):
      user = row_offset + u
      for item in items:
        tc.append([user, item])
    
    np.savez_compressed(file_name, np.asarray(tc))
    logging.info("Done saving to file %s.", file_name)