Allow YOLOv5 execution from arbitrary cwd (#4954)

* Allow YOLOv5 execution from arbitrary `cwd`

* Fix str bugs

export.py

@@ -34,6 +34,7 @@ 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
 
 from models.common import Conv
 from models.experimental import attempt_load