[Bug] add_edges() crashes if the input tensor is empty (#2100)

* [Bug] add_edges() crashes if the input tensor is empty

* lint

* fix

python/dgl/backend/mxnet/tensor.py

@@ -49,6 +49,8 @@ def tensor(data, dtype=None):
         if dtype is None:
             if isinstance(data, np.ndarray):
                 dtype = np.int32 if data.dtype == np.bool else data.dtype
+            elif len(data) == 0:
+                dtype = np.int64
             else:
                 dtype = np.int64 if isinstance(data[0], numbers.Integral) else np.float32
         return nd.array(data, dtype=dtype)

python/dgl/utils/checks.py

@@ -35,7 +35,8 @@ def prepare_tensor(g, data, name):
         ret = data
     else:
         data = F.tensor(data)
-        if F.dtype(data) not in (F.int32, F.int64):
+        if (not (F.ndim(data) > 0 and F.shape(data)[0] == 0) and        # empty tensor
+                F.dtype(data) not in (F.int32, F.int64)):
             raise DGLError('Expect argument "{}" to have data type int32 or int64,'
                            ' but got {}.'.format(name, F.dtype(data)))
         ret = F.copy_to(F.astype(data, g.idtype), g.device)

tests/compute/test_transform.py

@@ -1004,6 +1004,15 @@ def test_add_edges(idtype):
     u, v = g.edges(form='uv', order='eid')
     assert F.array_equal(u, F.tensor([0, 1, 0, 0, 0], dtype=idtype))
     assert F.array_equal(v, F.tensor([1, 2, 1, 1, 1], dtype=idtype))
+    g = dgl.add_edges(g, [], [])
+    g = dgl.add_edges(g, 0, [])
+    g = dgl.add_edges(g, [], 0)
+    assert g.device == F.ctx()
+    assert g.number_of_nodes() == 3
+    assert g.number_of_edges() == 5
+    u, v = g.edges(form='uv', order='eid')
+    assert F.array_equal(u, F.tensor([0, 1, 0, 0, 0], dtype=idtype))
+    assert F.array_equal(v, F.tensor([1, 2, 1, 1, 1], dtype=idtype))
 
     # node id larger than current max node id
     g = dgl.graph(([0, 1], [1, 2]), idtype=idtype, device=F.ctx())