1.为什么用nce:https://www.douban.com/note/637007135/
2.nce-loss训练的实现:http://www.jeyzhang.com/tensorflow-learning-notes-3.html
3.词向量:http://www.jeyzhang.com/tensorflow-learning-notes-3.html
4.用TensorFlow实现word2vec :https://blog.csdn.net/u014595019/article/details/54093161