fix export pretrained_model when it is None

ppcls/engine/engine.py

@@ -548,12 +548,15 @@ class Engine(object):
             "use_multilabel",
             False) or "ATTRMetric" in self.config["Metric"]["Eval"][0]
         model = ExportModel(self.config["Arch"], self.model, use_multilabel)
-        if self.config["Global"]["pretrained_model"].startswith("http"):
-            load_dygraph_pretrain_from_url(
-                model.base_model, self.config["Global"]["pretrained_model"])
-        else:
-            load_dygraph_pretrain(model.base_model,
-                                  self.config["Global"]["pretrained_model"])
+        if self.config["Global"]["pretrained_model"] is not None:
+            if self.config["Global"]["pretrained_model"].startswith("http"):
+                load_dygraph_pretrain_from_url(
+                    model.base_model,
+                    self.config["Global"]["pretrained_model"])
+            else:
+                load_dygraph_pretrain(
+                    model.base_model,
+                    self.config["Global"]["pretrained_model"])
 
         model.eval()