fix leaking imports in minimal testing

tests/utils/test_replay_buffer.py

@@ -20,6 +20,7 @@ from collections.abc import Callable
 import pytest
 
 pytest.importorskip("grpc")
+pytest.importorskip("datasets", reason="datasets is required (install lerobot[dataset])")
 
 import torch  # noqa: E402
 

tests/utils/test_visualization_utils.py

@@ -21,6 +21,8 @@ from types import SimpleNamespace
 import numpy as np
 import pytest
 
+pytest.importorskip("rerun", reason="rerun-sdk is required (install lerobot[viz])")
+
 from lerobot.types import TransitionKey
 from lerobot.utils.constants import OBS_STATE