Self-loops management in KNNGraph and RadiusGraph (#522)

* Add self-loop option to RadiusGraph and KNNGraph

tests/test_graph.py

@@ -67,8 +67,9 @@ def test_build_graph(x, pos):
         ),
     ],
 )
-def test_build_radius_graph(x, pos):
-    graph = RadiusGraph(x=x, pos=pos, radius=0.5)
+@pytest.mark.parametrize("loop", [True, False])
+def test_build_radius_graph(x, pos, loop):
+    graph = RadiusGraph(x=x, pos=pos, radius=0.5, loop=loop)
     assert hasattr(graph, "x")
     assert hasattr(graph, "pos")
     assert hasattr(graph, "edge_index")
@@ -84,6 +85,15 @@ def test_build_radius_graph(x, pos):
         assert graph.pos.labels == pos.labels
     else:
         assert isinstance(graph.pos, torch.Tensor)
+    if not loop:
+        assert (
+            len(
+                torch.nonzero(
+                    graph.edge_index[0] == graph.edge_index[1], as_tuple=True
+                )[0]
+            )
+            == 0
+        )  # Detect self loops
 
 
 @pytest.mark.parametrize(
@@ -168,8 +178,9 @@ def test_build_radius_graph_custom_edge_attr(x, pos):
         ),
     ],
 )
-def test_build_knn_graph(x, pos):
-    graph = KNNGraph(x=x, pos=pos, neighbours=2)
+@pytest.mark.parametrize("loop", [True, False])
+def test_build_knn_graph(x, pos, loop):
+    graph = KNNGraph(x=x, pos=pos, neighbours=2, loop=loop)
     assert hasattr(graph, "x")
     assert hasattr(graph, "pos")
     assert hasattr(graph, "edge_index")
@@ -186,6 +197,15 @@ def test_build_knn_graph(x, pos):
     else:
         assert isinstance(graph.pos, torch.Tensor)
     assert graph.edge_attr is None
+    self_loops = len(
+        torch.nonzero(
+            graph.edge_index[0] == graph.edge_index[1], as_tuple=True
+        )[0]
+    )
+    if loop:
+        assert self_loops != 0
+    else:
+        assert self_loops == 0
 
 
 @pytest.mark.parametrize(