[cherry-pick]remove unnecessary parameters in IouLoss (#1258)

* fix deadlink in docs/tutorials/INSTALL_cn.md, slim/README.md and slim/extensions/distill_pruned_model/README.md

ppdet/modeling/losses/iou_loss.py

@@ -235,4 +235,5 @@ class IouLoss(object):
             dtype=numpy_array.dtype,
             default_initializer=NumpyArrayInitializer(numpy_array))
         paddle_array.stop_gradient = True
+        paddle_array.persistable = False
         return paddle_array