Update tutorials (#463)

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Co-authored-by: Dario Coscia <93731561+dario-coscia@users.noreply.github.com>

pina/label_tensor.py

@@ -479,10 +479,9 @@ class LabelTensor(torch.Tensor):
 
         # Retrieve selected tensor and labels
         selected_tensor = super().__getitem__(index)
-        if hasattr(self, "_labels"):
-            original_labels=self._labels
-        else:
+        if not hasattr(self, "_labels"):
             return selected_tensor
+
         original_labels = self._labels
         updated_labels = copy(original_labels)