seq2seq升级TF1.5后_Linear报错

解决TF升级到1.5之后seq2seq.py出现的引用报错：

1.4时候使用rnn_cell_impl的_Linear没有问题的，TF升级到1.5之后这一行就运行不过去了，查到的方法是引用core_rnn_cell下的_Linear.

 from tensorflow.contrib.rnn.python.ops import core_rnn_cell
 from tensorflow.python.framework import dtypes
 from tensorflow.python.framework import ops
 from tensorflow.python.ops import array_ops
 from tensorflow.python.ops import control_flow_ops
 from tensorflow.python.ops import embedding_ops
 from tensorflow.python.ops import math_ops
 from tensorflow.python.ops import nn_ops
 from tensorflow.python.ops import rnn
 from tensorflow.python.ops import rnn_cell_impl
 from tensorflow.python.ops import variable_scope
 from tensorflow.python.util import nest

 # TODO(ebrevdo): Remove once _linear is fully deprecated.
 Linear = core_rnn_cell._Linear  # pylint: disable=protected-access,invalid-name