fix leaking imports in minimal testing

tests/datasets/test_delta_timestamps.py

@@ -13,6 +13,8 @@
 # limitations under the License.
 import pytest
 
+pytest.importorskip("datasets", reason="datasets is required (install lerobot[dataset])")
+
 from lerobot.datasets.feature_utils import (
     check_delta_timestamps,
     get_delta_indices,