fix: 🐛 fix bug when training object detection (#1254)

* fix: 🐛 fix bug when training object detection

* comment: add comments to make it clearer

doctr/datasets/datasets/base.py

@@ -55,7 +55,12 @@ def __getitem__(self, index: int) -> Tuple[Any, Any]:
             img = self.img_transforms(img)
 
         if self.sample_transforms is not None:
-            if isinstance(target, dict) and all([isinstance(item, np.ndarray) for item in target.values()]):
+            # Conditions to assess it is detection model with multiple classes and avoid confusion with other tasks.
+            if (
+                isinstance(target, dict)
+                and all([isinstance(item, np.ndarray) for item in target.values()])
+                and set(target.keys()) != {"boxes", "labels"}  # avoid confusion with obj detection target
+            ):
                 img_transformed = copy_tensor(img)
                 for class_name, bboxes in target.items():
                     img_transformed, target[class_name] = self.sample_transforms(img, bboxes)