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>

detect.py

 """
 
 import argparse
+import os
 import sys
 from pathlib import Path
 
 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
 
 from models.experimental import attempt_load
 from utils.datasets import LoadImages, LoadStreams

export.py

 """
 
 import argparse
+import os
 import subprocess
 import sys
 import time
 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
 
 from models.common import Conv
 from models.experimental import attempt_load

train.py

 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

val.py

 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
 
 from models.experimental import attempt_load
 from utils.datasets import create_dataloader