standalone val (#56)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

ultralytics/yolo/data/dataset.py

@@ -208,6 +208,9 @@ class ClassificationDataset(torchvision.datasets.ImageFolder):
             sample = self.torch_transforms(im)
         return OrderedDict(img=sample, cls=j)
 
+    def __len__(self) -> int:
+        return len(self.samples)
+
 
 # TODO: support semantic segmentation
 class SemanticDataset(BaseDataset):