Created by: jacquesqiao

nce_op:

• Make nce support more distribution https://github.com/PaddlePaddle/Paddle/pull/13549
• support sparse update https://github.com/PaddlePaddle/Paddle/pull/14469

hsigmoid_op:

• Enhance hierarchical sigmoid op https://github.com/PaddlePaddle/Paddle/pull/14352
• support sparse udpate

models

skip-gram word2vec models https://github.com/PaddlePaddle/models/pull/1448