中身を確認
  ・python -m pickle mnist.pkl     {'test_img': array([[0, 0, 0, ..., 0, 0, 0],      [0, 0, 0, ..., 0, 0, 0],      [0, 0, 0, ..., 0, 0, 0],      ...,      [0, 0, 0, ..., 0, 0, 0],      [0, 0, 0, ..., 0, 0, 0],      [0, 0, 0, ..., 0, 0, 0]], dtype=uint8),      'test_label': array([7, 2, 1, ..., 4, 5, 6], dtype=uint8),      'train_img': array([[0, 0, 0, ..., 0, 0, 0],      [0, 0, 0, ..., 0, 0, 0],      [0, 0, 0, ..., 0, 0, 0],      ...,      [0, 0, 0, ..., 0, 0, 0],      [0, 0, 0, ..., 0, 0, 0],      [0, 0, 0, ..., 0, 0, 0]], dtype=uint8),      'train_label': array([5, 0, 4, ..., 5, 6, 8], dtype=uint8)}