Fix segment_reduce() ignoring tailing 0 segments (#2228)

Co-authored-by: Quan (Andy) Gan <coin2028@hotmail.com>

python/dgl/ops/segment.py

@@ -50,7 +50,8 @@ def segment_reduce(seglen, value, reducer='sum'):
     if len(u) != len(v):
         raise DGLError("Invalid seglen array:", seglen,
                        ". Its summation must be equal to value.shape[0].")
-    g = convert.heterograph({('_U', '_E', '_V'): (u, v)})
+    num_nodes = {'_U': len(u), '_V': len(seglen)}
+    g = convert.heterograph({('_U', '_E', '_V'): (u, v)}, num_nodes_dict=num_nodes)
     g.srcdata['h'] = value
     g.update_all(fn.copy_u('h', 'm'), getattr(fn, reducer)('m', 'h'))
     return g.dstdata['h']