CUDA option for labeltensor (#23)

* fix cuda device for labeltensor

pina/label_tensor.py

@@ -116,7 +116,10 @@ class LabelTensor(torch.Tensor):
 
         new_data = self[:, indeces].float()
         new_labels = [self.labels[idx] for idx in indeces]
-        extracted_tensor = LabelTensor(new_data, new_labels)
+
+        extracted_tensor = new_data.as_subclass(LabelTensor)
+        extracted_tensor.labels = new_labels
+
 
         return extracted_tensor
 
@@ -150,9 +153,15 @@ class LabelTensor(torch.Tensor):
                 tensor2.repeat_interleave(n1, dim=0),
                 labels=tensor2.labels)
             new_tensor = torch.cat((tensor1, tensor2), dim=1)
-        return LabelTensor(new_tensor, new_labels)
+
+        new_tensor = new_tensor.as_subclass(LabelTensor)
+        new_tensor.labels = new_labels
+        return new_tensor
 
     def __str__(self):
-        s = f'labels({str(self.labels)})\n'
+        if hasattr(self, 'labels'):
+            s = f'labels({str(self.labels)})\n'
+        else:
+            s = 'no labels\n'
         s += super().__str__()
         return s