[BugFix] fix crash due to incorrect dtype in dgl.to_block() (#4487)

* [BugFix] fix crash due to incorrect dtype in dgl.to_block()

* fix test failure in TF

python/dgl/transforms/functional.py

@@ -2317,7 +2317,7 @@ def to_block(g, dst_nodes=None, include_dst_in_src=True, src_nodes=None):
                 'Graph has more than one node type; please specify a dict for src_nodes.')
         src_nodes = {g.ntypes[0]: src_nodes}
         src_node_ids = [
-            F.copy_to(F.tensor(src_nodes.get(ntype, []), dtype=g._idtype_str), \
+            F.copy_to(F.tensor(src_nodes.get(ntype, []), dtype=g.idtype), \
                 F.to_backend_ctx(g._graph.ctx)) \
             for ntype in g.ntypes]
         src_node_ids_nd = [F.to_dgl_nd(nodes) for nodes in src_node_ids]

tests/compute/test_transform.py

@@ -913,6 +913,17 @@ def test_to_block(idtype):
             else:
                 check(g, bg, ntype, etype, None, include_dst_in_src)
 
+    # homogeneous graph
+    g = dgl.graph((F.tensor([1, 2], dtype=idtype), F.tensor([2, 3], dtype=idtype)))
+    dst_nodes = F.tensor([3, 2], dtype=idtype)
+    bg = dgl.to_block(g, dst_nodes=dst_nodes)
+    check(g, bg, '_N', '_E', dst_nodes)
+
+    src_nodes = bg.srcnodes['_N'].data[dgl.NID]
+    bg = dgl.to_block(g, dst_nodes=dst_nodes, src_nodes=src_nodes)
+    check(g, bg, '_N', '_E', dst_nodes)
+
+    # heterogeneous graph
     g = dgl.heterograph({
         ('A', 'AA', 'A'): ([0, 2, 1, 3], [1, 3, 2, 4]),
         ('A', 'AB', 'B'): ([0, 1, 3, 1], [1, 3, 5, 6]),