fix leaking imports in minimal testing

tests/datasets/test_dataset_writer.py

@@ -23,6 +23,8 @@ import pytest
 import torch
 from PIL import Image
 
+pytest.importorskip("datasets", reason="datasets is required (install lerobot[dataset])")
+
 from lerobot.datasets.dataset_writer import _encode_video_worker
 from lerobot.datasets.lerobot_dataset import LeRobotDataset
 from lerobot.datasets.utils import DEFAULT_IMAGE_PATH