[Docs] update data element tutorials (#431)

* structure tutorials

* refine data element docs

* modify introduce

* fix comment

* fix comment

* fix comment

mmengine/structures/label_data.py

@@ -22,7 +22,7 @@ class LabelData(BaseDataElement):
         assert isinstance(onehot, torch.Tensor)
         if (onehot.ndim == 1 and onehot.max().item() <= 1
                 and onehot.min().item() >= 0):
-            return onehot.nonzero().squeeze()
+            return onehot.nonzero().squeeze(-1)
         else:
             raise ValueError(
                 'input is not one-hot and can not convert to label')

tests/test_structures/test_label_data.py

@@ -44,6 +44,11 @@ class TestLabelData(TestCase):
         label = LabelData.onehot_to_label(onehot)
         assert (label == item).all()
         assert label.device == item.device
+        item = torch.tensor([2])
+        onehot = LabelData.label_to_onehot(item, num_classes=10)
+        label = LabelData.onehot_to_label(onehot)
+        assert label == item
+        assert label.device == item.device
 
     @pytest.mark.skipif(
         not torch.cuda.is_available(), reason='GPU is required!')