Update sys.path.append(str(ROOT)) (#4852)

* Update `sys.path.append(str(ROOT))`

* Cleanup

models/tf.py

@@ -17,8 +17,9 @@ from copy import deepcopy
 from pathlib import Path
 
 FILE = Path(__file__).resolve()
-ROOT = FILE.parents[1]  # yolov5/ dir
-sys.path.append(ROOT.as_posix())  # add yolov5/ to path
+ROOT = FILE.parents[1]  # YOLOv5 root directory
+if str(ROOT) not in sys.path:
+    sys.path.append(str(ROOT))  # add ROOT to PATH
 
 import numpy as np
 import tensorflow as tf