fix gc bug

test=develop

paddle/fluid/framework/executor.cc

@@ -100,7 +100,7 @@ static void DeleteUnusedTensors(
           continue;
         }
         auto* var = scope.FindVar(name);
-        if (var != nullptr) {
+        if (var == nullptr) {
           continue;
         }
 

python/paddle/fluid/tests/unittests/test_eager_deletion_dynamic_rnn_base.py

@@ -39,6 +39,7 @@ def train(network, use_cuda, use_parallel_executor, batch_size=32, pass_num=2):
     label = fluid.layers.data(name="label", shape=[1], dtype="int64")
 
     cost = network(data, label, len(word_dict))
+    cost.persistable = True
     optimizer = fluid.optimizer.Adagrad(learning_rate=0.2)
     optimizer.minimize(cost)