Fix net_drawer

test=develop

python/paddle/fluid/net_drawer.py

@@ -26,7 +26,7 @@ logger = logging.getLogger(__name__)
 logger.setLevel(logging.INFO)
 
 try:
-    from .graphviz import Digraph
+    from .graphviz import Graph
 except ImportError:
     logger.info(
         'Cannot import graphviz, which is required for drawing a network. This '
@@ -112,7 +112,7 @@ def draw_graph(startup_program, main_program, **kwargs):
     filename = kwargs.get("filename")
     if filename == None:
         filename = str(graph_id) + ".gv"
-    g = Digraph(
+    g = Graph(
         name=str(graph_id),
         filename=filename,
         graph_attr=GRAPH_STYLE,