bug #60: display message when yolo is used but dependencies not installed

src/reachy_mini_conversation_demo/vision/yolo_head_tracker.py

@@ -3,8 +3,15 @@ import logging
 from typing import Tuple, Optional
 
 import numpy as np
-from supervision import Detections
-from ultralytics import YOLO
+
+
+try:
+    from supervision import Detections
+    from ultralytics import YOLO
+except ImportError as e:
+    raise ImportError(
+        "To use YOLO head tracker, please install the extra dependencies: pip install '.[yolo_vision]'"
+    ) from e
 from huggingface_hub import hf_hub_download