fix trainer.py load_weights_sde when reid mode is None (#7604)

* fix trainer.py load_weights_sde when reid mode is None

* fix trainer.py load_weights_sde

ppdet/engine/trainer.py

@@ -399,7 +399,8 @@ class Trainer(object):
     def load_weights_sde(self, det_weights, reid_weights):
         if self.model.detector:
             load_weight(self.model.detector, det_weights)
-            load_weight(self.model.reid, reid_weights)
+            if self.model.reid:
+                load_weight(self.model.reid, reid_weights)
         else:
             load_weight(self.model.reid, reid_weights)