Fixing Laplacian operator for vector fields (#380)

* fix laplacian and tests

tests/test_operators.py

@@ -52,15 +52,26 @@ def test_div_vector_output():
 
 
 def test_laplacian_scalar_output():
-    laplace_tensor_v = laplacian(tensor_s, inp, components=['a'], d=['x', 'y'])
-    assert laplace_tensor_v.shape == tensor_s.shape
+    laplace_tensor_s = laplacian(tensor_s, inp, components=['a'], d=['x', 'y'])
+    assert laplace_tensor_s.shape == tensor_s.shape
+    assert laplace_tensor_s.labels == [f"dd{tensor_s.labels[0]}"]
+    true_val = 4*torch.ones_like(laplace_tensor_s)
+    assert all((laplace_tensor_s - true_val == 0).flatten())
 
 
 def test_laplacian_vector_output():
     laplace_tensor_v = laplacian(tensor_v, inp)
     assert laplace_tensor_v.shape == tensor_v.shape
+    assert laplace_tensor_v.labels == [
+        f'dd{i}' for i in tensor_v.labels
+    ]
     laplace_tensor_v = laplacian(tensor_v,
                                  inp,
                                  components=['a', 'b'],
                                  d=['x', 'y'])
     assert laplace_tensor_v.shape == tensor_v.extract(['a', 'b']).shape
+    assert laplace_tensor_v.labels == [
+        f'dd{i}' for i in ['a', 'b']
+    ]
+    true_val = 2*torch.ones_like(tensor_v.extract(['a', 'b']))
+    assert all((laplace_tensor_v - true_val == 0).flatten())