Polish code

test=develop

paddle/fluid/pybind/imperative.cc

@@ -24,9 +24,8 @@ namespace pybind {
 void BindTracer(pybind11::module *m) {
   pybind11::class_<imperative::Tracer>(*m, "Tracer", "")
       .def("__init__",
-           [](imperative::Tracer &self, framework::BlockDesc *root_block,
-              framework::BlockDesc *startup_block) {
-             new (&self) imperative::Tracer(root_block, startup_block);
+           [](imperative::Tracer &self, framework::BlockDesc *root_block) {
+             new (&self) imperative::Tracer(root_block);
            })
       .def("trace", &imperative::Tracer::Trace)
       .def("get_scope", &imperative::Tracer::GetScope,

python/paddle/fluid/imperative/base.py

@@ -28,8 +28,7 @@ def enabled():
 def guard():
     train = framework.Program()
     startup = framework.Program()
-    tracer = core.Tracer(train.current_block().desc,
-                         startup.current_block().desc)
+    tracer = core.Tracer(train.current_block().desc)
     with framework.program_guard(train, startup):
         with framework.unique_name.guard():
             with framework._imperative_guard(tracer):