Fix ResNet assertion error when `feature_maps` is an integer (#2508)

PaddleCV/object_detection/ppdet/modeling/backbones/resnet.py

@@ -56,6 +56,9 @@ class ResNet(object):
                  feature_maps=[2, 3, 4, 5]):
         super(ResNet, self).__init__()
 
+        if isinstance(feature_maps, Integral):
+            feature_maps = [feature_maps]
+
         assert depth in [18, 34, 50, 101, 152], \
             "depth {} not in [18, 34, 50, 101, 152]"
         assert variant in ['a', 'b', 'c', 'd'], "invalid ResNet variant"
@@ -70,8 +73,6 @@ class ResNet(object):
         self.freeze_norm = freeze_norm
         self.variant = variant
         self._model_type = 'ResNet'
-        if isinstance(feature_maps, Integral):
-            feature_maps = [feature_maps]
         self.feature_maps = feature_maps
         self.depth_cfg = {
             18: ([2, 2, 2, 2], self.basicblock),