Returns SplitGenerators.

def _split_generators(self, dl_manager):
    """Returns SplitGenerators."""
    # Download the full MNIST Database
    filenames = {
        "train_data": _MNIST_TRAIN_DATA_FILENAME,
        "train_labels": _MNIST_TRAIN_LABELS_FILENAME,
        "test_data": _MNIST_TEST_DATA_FILENAME,
        "test_labels": _MNIST_TEST_LABELS_FILENAME,
    }
    mnist_files = dl_manager.download_and_extract(
        {k: urllib.parse.urljoin(self.URL, v) for k, v in filenames.items()})

    # MNIST provides TRAIN and TEST splits, not a VALIDATION split, so we only
    # write the TRAIN and TEST splits to disk.
    return [
        tfds.core.SplitGenerator(
            name=tfds.Split.TRAIN,
            num_shards=10,
            gen_kwargs=dict(
                num_examples=_TRAIN_EXAMPLES,
                data_path=mnist_files["train_data"],
                label_path=mnist_files["train_labels"],
            )),
        tfds.core.SplitGenerator(
            name=tfds.Split.TEST,
            num_shards=1,
            gen_kwargs=dict(
                num_examples=_TEST_EXAMPLES,
                data_path=mnist_files["test_data"],
                label_path=mnist_files["test_labels"],
            )),
    ]