reduce ut time

test=develop

paddle/fluid/imperative/layer.h

@@ -127,10 +127,7 @@ class VarBase {
 
  public:
   virtual ~VarBase() {
-    if (block_ && !persistable_) {
-      block_->RemoveVar(name_);
-    }
-
+    // TODO(minqiyang): remove var desc from block desc
     if (var_) {
       delete var_;
       var_ = nullptr;

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

@@ -243,7 +243,7 @@ class TestImperativePtbRnn(unittest.TestCase):
             dy_loss = None
             last_hidden = None
             last_cell = None
-            batch_num = 200
+            batch_num = 50
 
             for i in range(batch_num):
                 x_data = np.arange(12).reshape(4, 3).astype('int64')

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

@@ -231,7 +231,7 @@ class TestImperativeResnet(unittest.TestCase):
         seed = 90
 
         batch_size = train_parameters["batch_size"]
-        batch_num = 50
+        batch_num = 20
         with fluid.imperative.guard():
             fluid.default_startup_program().random_seed = seed
             fluid.default_main_program().random_seed = seed