Merge pull request #1382 from chengduoZH/release/0.15.0

Fix transformer model

fluid/neural_machine_translation/transformer/model.py

@@ -203,6 +203,7 @@ def prepare_encoder(src_word,
         size=[src_max_len, src_emb_dim],
         param_attr=fluid.ParamAttr(
             name=pos_enc_param_name, trainable=False))
+    src_pos_enc.stop_gradient = True
     enc_input = src_word_emb + src_pos_enc
     return layers.dropout(
         enc_input,