Sidestep os.path.relpath() Windows bug (ultralytics#7158)

* Sidestep os.path.relpath() Windows bug

os.path.relpath() seems to have a major bug on Windows due to Windows horrible path handling. This fix attempts to sidestep the issue.

```
File "C:\Users\mkokg/.cache\torch\hub\ultralytics_yolov5_master\export.py", line 64, in
ROOT = Path(os.path.relpath(ROOT, Path.cwd())) # relative
File "C:\Users\mkokg\AppData\Local\Programs\Python\Python310\lib\ntpath.py", line 718, in relpath
raise ValueError("path is on mount %r, start on mount %r" % (
ValueError: path is on mount 'C:', start on mount 'D:'
```

* Update yolo.py

* Update yolo.py

* Update yolo.py

* Update export.py

export.py

@@ -61,7 +61,8 @@
 ROOT = FILE.parents[0]  # YOLOv5 root directory
 if str(ROOT) not in sys.path:
     sys.path.append(str(ROOT))  # add ROOT to PATH
-ROOT = Path(os.path.relpath(ROOT, Path.cwd()))  # relative
+if platform.system() != 'Windows':
+    ROOT = Path(os.path.relpath(ROOT, Path.cwd()))  # relative
 
 from models.common import Conv
 from models.experimental import attempt_load

models/yolo.py

@@ -7,6 +7,8 @@
 """
 
 import argparse
+import os
+import platform
 import sys
 from copy import deepcopy
 from pathlib import Path
@@ -15,7 +17,8 @@
 ROOT = FILE.parents[1]  # 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
+if platform.system() != 'Windows':
+    ROOT = Path(os.path.relpath(ROOT, Path.cwd()))  # relative
 
 from models.common import *
 from models.experimental import *