docs: def random_partition(n, n_data):
      """return n random rows of data (and also the other len(data)-n rows)"""
      all_idxs = np.arange(n_data)
      np.random.shuffle(all_idxs)
      idxs1 = all_idxs[:n]
      idxs2 = all_idxs[n:]
      return idxs1, idxs2