Fix img_pool_layer bug.

python/paddle/trainer_config_helpers/layers.py

@@ -2607,15 +2607,15 @@ def img_pool_layer(input,
         assert input.num_filters is not None
         num_channels = input.num_filters
 
-    assert type(pool_type) in [AvgPooling, MaxPooling, CudnnAvgPooling,
-                               CudnnMaxPooling], \
-        "only (Cudnn)AvgPooling, (Cudnn)MaxPooling are supported"
-
     if pool_type is None:
         pool_type = MaxPooling()
     elif isinstance(pool_type, AvgPooling):
         pool_type.name = 'avg'
 
+    assert type(pool_type) in [AvgPooling, MaxPooling, CudnnAvgPooling,
+                               CudnnMaxPooling], \
+        "only (Cudnn)AvgPooling, (Cudnn)MaxPooling are supported"
+
     type_name = pool_type.name + '-projection' \
         if (
         isinstance(pool_type, AvgPooling) or isinstance(pool_type, MaxPooling)) \