fix trainer load parameters (#12817)

python/paddle/fluid/trainer.py

@@ -285,6 +285,7 @@ class Trainer(object):
             self._load_checkpoint()
 
         if param_path and os.path.isdir(param_path):
+            with self._prog_and_scope_guard():
                 # load params from param_path into scope
                 io.load_persistables(
                     executor=exe,