fix ProgramToGraph

when while_grad, it writes multiple @EMPTY@ with no VarDesc.

paddle/fluid/framework/ir/graph.cc

@@ -117,6 +117,15 @@ Graph::Graph(const ProgramDesc &program) : program_(program) {
     }
     // For output args, always create a new var.
     for (auto &each_var_name : op->OutputArgumentNames()) {
+      ir::Node *var = nullptr;
+      if (all_vars.count(each_var_name) != 0) {
+        var = CreateVarNode(all_vars.at(each_var_name));
+      } else {
+        // Operation output vars can be @EMPTY@. For example, while_grad
+        // can have multi @EMPTY@ outputs with no VarDesc.
+        // TODO(panyx0718): Add a test.
+        var = CreateEmptyNode(each_var_name, ir::Node::Type::kVariable);
+      }
       ir::Node *var = CreateVarNode(all_vars.at(each_var_name));
       var_nodes[each_var_name].push_back(var);
       node->outputs.push_back(var);
@@ -208,7 +217,8 @@ Graph::Graph(const ProgramDesc &program) : program_(program) {
       // Add write after write dependence
       ir::Node *upstream_op =
           (*it_old)->inputs.empty() ? nullptr : (*it_old)->inputs[0];
-      if (upstream_op) {
+      // TODO(zcd): Add a test.
+      if (upstream_op && upstream_op != write_op) {
         ir::Node *dep_var = CreateControlDepVar();
         write_op->inputs.push_back(dep_var);
         upstream_op->outputs.push_back(dep_var);