fix leaking imports in minimal testing

tests/conftest.py

@@ -17,24 +17,39 @@
 import traceback
 
 import pytest
-from serial import SerialException
 
 from lerobot.configs.types import FeatureType, PipelineFeatureType, PolicyFeature
+from lerobot.utils.import_utils import is_package_available
 from tests.utils import DEVICE
 
-# Import fixture modules as plugins
+# Import fixture modules as plugins.
+# Fixtures that depend on optional packages are only registered when those packages are available,
+# so that tests can be collected and run even with a minimal install.
 pytest_plugins = [
-    "tests.fixtures.dataset_factories",
-    "tests.fixtures.files",
-    "tests.fixtures.hub",
     "tests.fixtures.optimizers",
 ]
 
+if is_package_available("datasets"):
+    pytest_plugins += [
+        "tests.fixtures.dataset_factories",
+        "tests.fixtures.files",
+        "tests.fixtures.hub",
+    ]
+
 
 def pytest_collection_finish():
     print(f"\nTesting with {DEVICE=}")
 
 
+def _is_serial_exception(exc: Exception) -> bool:
+    """Check if an exception is a SerialException without requiring pyserial."""
+    if not is_package_available("pyserial", import_name="serial"):
+        return False
+    from serial import SerialException
+
+    return isinstance(exc, SerialException)
+
+
 def _check_component_availability(component_type, available_components, make_component):
     """Generic helper to check if a hardware component is available"""
     if component_type not in available_components:
@@ -53,7 +68,7 @@ def _check_component_availability(component_type, available_components, make_com
 
         if isinstance(e, ModuleNotFoundError):
             print(f"\nInstall module '{e.name}'")
-        elif isinstance(e, SerialException):
+        elif _is_serial_exception(e):
             print("\nNo physical device detected.")
         elif isinstance(e, ValueError) and "camera_index" in str(e):
             print("\nNo physical camera detected.")