refactor(imports): enforce guard pattern (#3382)

* refactor(imports): enforce guard pattern

* fix(tests): skip reachy2 if not installed

* Address review feedback

src/lerobot/policies/groot/modeling_groot.py

@@ -43,6 +43,7 @@ from torch import Tensor
 
 from lerobot.configs import FeatureType, PolicyFeature
 from lerobot.utils.constants import ACTION, OBS_IMAGES
+from lerobot.utils.import_utils import require_package
 
 from ..pretrained import PreTrainedPolicy
 from .configuration_groot import GrootConfig
@@ -59,6 +60,7 @@ class GrootPolicy(PreTrainedPolicy):
 
     def __init__(self, config: GrootConfig, **kwargs):
         """Initialize Groot policy wrapper."""
+        require_package("transformers", extra="groot")
         super().__init__(config)
         config.validate_features()
         self.config = config