Returns SplitGenerators.

def _split_generators(self, dl_manager):
    """Returns SplitGenerators."""
    root_url = "http://images.cocodataset.org/"
    urls = {
        # Train/validation set
        "train_images": "zips/train2014.zip",
        "val_images": "zips/val2014.zip",
        "trainval_annotations": "annotations/annotations_trainval2014.zip",
        # Testing set (no annotations) (2014)
        "test_images": "zips/test2014.zip",
        "test_annotations": "annotations/image_info_test2014.zip",
        # Testing set (no annotations) (2015)
        "test2015_images": "zips/test2015.zip",
        "test2015_annotations": "annotations/image_info_test2015.zip",
    }
    extracted_paths = dl_manager.download_and_extract({
        key: root_url + url for key, url in urls.items()
    })

    return [
        tfds.core.SplitGenerator(
            name=tfds.Split.TRAIN,
            num_shards=10,
            gen_kwargs=dict(
                image_dir=extracted_paths["train_images"],
                annotation_dir=extracted_paths["trainval_annotations"],
                split_type="train2014",
            )),
        tfds.core.SplitGenerator(
            name=tfds.Split.VALIDATION,
            num_shards=10,
            gen_kwargs=dict(
                image_dir=extracted_paths["val_images"],
                annotation_dir=extracted_paths["trainval_annotations"],
                split_type="val2014",
            )),
        # Warning: Testing split only contains the images without any annotation
        tfds.core.SplitGenerator(
            name=tfds.Split.TEST,
            num_shards=10,
            gen_kwargs=dict(
                image_dir=extracted_paths["test_images"],
                annotation_dir=extracted_paths["test_annotations"],
                split_type="test2014",
                has_annotation=False,
            )),
        tfds.core.SplitGenerator(
            name="test2015",
            num_shards=10,
            gen_kwargs=dict(
                image_dir=extracted_paths["test2015_images"],
                annotation_dir=extracted_paths["test2015_annotations"],
                split_type="test2015",
                has_annotation=False,
            )),
    ]