Replace v2.dataset with dataset in Transformer

fluid/neural_machine_translation/transformer/infer.py

 import numpy as np
 
-import paddle.v2 as paddle
+import paddle
 import paddle.fluid as fluid
 
 import model

fluid/neural_machine_translation/transformer/train.py

 import os
 import numpy as np
 
-import paddle.v2 as paddle
+import paddle
 import paddle.fluid as fluid
 
 from model import transformer, position_encoding_init