Fix bug in handling labels with LabelTensor (#460)

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Co-authored-by: Filippo Olivo <filippo@filippoolivo.com>

pina/label_tensor.py

@@ -448,6 +448,9 @@ class LabelTensor(torch.Tensor):
 
         # Retrieve selected tensor and labels
         selected_tensor = super().__getitem__(index)
+        if not hasattr(self, "_labels"):
+            return selected_tensor
+
         original_labels = self._labels
         updated_labels = copy(original_labels)