fix default value. test=develop

paddle/fluid/framework/details/memory_optimize_pass.cc

@@ -236,8 +236,12 @@ void MemoryOptimizePass::RenameVarInGraphDesc(const std::string& var,
     auto* op_desc = op->Op();
     op_desc->RenameInput(var, cache_var);
     op_desc->RenameOutput(var, cache_var);
-    if (op_desc->Block() != nullptr && op_desc->Block()->HasVar(var)) {
+    if (op_desc->Block() != nullptr) {
       op_desc->Block()->RemoveVar(var);
+    } else {
+      LOG(WARNING) << "op " << op->Name() << " not know its block."
+                   << "Is the op_desc created without block pointer? "
+                   << "Can not find " << var << " in Block(0)";
     }
     op_desc->Flush();
   }