ultralytics 8.0.19 seg/det dataset warning and DDP-cls/seg fixes (#595)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: 曾逸夫（Zeng Yifu） <41098760+Zengyf-CVer@users.noreply.github.com>
Co-authored-by: Laughing <61612323+Laughing-q@users.noreply.github.com>

ultralytics/yolo/data/dataloaders/v5loader.py

@@ -611,6 +611,8 @@ class LoadImagesAndLabels(Dataset):
 
     def cache_labels(self, path=Path('./labels.cache'), prefix=''):
         # Cache dataset labels, check images and read shapes
+        if path.exists():
+            path.unlink()  # remove *.cache file if exists
         x = {}  # dict
         nm, nf, ne, nc, msgs = 0, 0, 0, 0, []  # number missing, found, empty, corrupt, messages
         desc = f"{prefix}Scanning {path.parent / path.stem}..."