fix slicing for LabelTensor (#167)

* fix slicing for LabelTensor
* Update testing_pr.yml for solving python3.1 error

tests/test_label_tensor.py

@@ -73,7 +73,7 @@ def test_merge():
     tensor_bc = tensor_b.append(tensor_c)
     assert torch.allclose(tensor_bc, tensor.extract(['b', 'c']))
 
-def test_merge():
+def test_merge2():
     tensor = LabelTensor(data, labels)
     tensor_b = tensor.extract('b')
     tensor_c = tensor.extract('c')
@@ -87,4 +87,18 @@ def test_getitem():
     tensor_view = tensor[:5]
 
     assert tensor_view.labels == labels
-    assert torch.allclose(tensor_view, data[:5])
+    assert torch.allclose(tensor_view, data[:5])
+
+def test_slice():
+    tensor = LabelTensor(data, labels)
+    tensor_view = tensor[:5, :2]
+    assert tensor_view.labels == labels[:2]
+    assert torch.allclose(tensor_view, data[:5, :2])
+
+    tensor_view2 = tensor[3]
+    assert tensor_view2.labels == labels
+    assert torch.allclose(tensor_view2, data[3])
+
+    tensor_view3 = tensor[:, 2]
+    assert tensor_view3.labels == labels[2]
+    assert torch.allclose(tensor_view3, data[:, 2].reshape(-1, 1))