[Dy2St]Fix cascade_rcnn export model problem (#6468)

* [Dy2St]Fix cascade_rcnn export model problem

* [Dy2St]Fix cascade_rcnn export model problem

ppdet/modeling/heads/cascade_head.py

@@ -300,6 +300,12 @@ class CascadeHead(BBoxHead):
         num_prop = []
         for p in proposals:
             num_prop.append(p.shape[0])
+
+        # NOTE(dev): num_prob will be tagged as LoDTensorArray because it
+        # depends on batch_size under @to_static. However the argument
+        # num_or_sections in paddle.split does not support LoDTensorArray,
+        # so we use [-1] to replace it and whitout lossing correctness.
+        num_prop = [-1] if len(num_prop) == 1 else num_prop
         return pred_bbox.split(num_prop)
 
     def get_prediction(self, head_out_list):