Fix ROOT as relative path (#5129)

* use os.path.relpath instead of relative_to

* use os.path.relpath instead of relative_to

* Remove os.path from val.py

* Remove os.path from train.py

* Update detect.py import to os

* Update export.py import to os

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

train.py

@@ -30,7 +30,7 @@ FILE = Path(__file__).resolve()
 ROOT = FILE.parents[0]  # YOLOv5 root directory
 if str(ROOT) not in sys.path:
     sys.path.append(str(ROOT))  # add ROOT to PATH
-ROOT = ROOT.relative_to(Path.cwd())  # relative
+ROOT = Path(os.path.relpath(ROOT, Path.cwd()))  # relative
 
 import val  # for end-of-epoch mAP
 from models.experimental import attempt_load