* -1->None (#823)

* unify paddle 1.6 api in understand_sentiment (#824)


* unify paddle 1.6 api in understand_sentiment

* Upgrade w2v & srl's api (#828)

* Upgrade w2v & srl's api

* Upgrade label semantic roles api

* Rewrite 08.machine_translation using Paddle-1.6 apis. (#826)

* Rewrite 08.machine_translation using Paddle-1.6 apis.

* Delete the old train.py in 08.machine_translation

* Update train.py to seq2seq.py in README_cn 08.machine_translation.

* Fix the print content of seq2seq.py.

* Update code format in README_cn of 08.machine_translation.

* add 1.6 requirement (#830)

* fix bugs in Readme.md of 04 and 07 (#831) (#832)

* Polish optimizer name in 04.word2vec (#841) (#842)

* change opt name in word2vec

* update index.cn.html

* Cherry pick for paddle 1.6 (#843)
Co-authored-by: Nceci3 <ceci3@users.noreply.github.com>
Co-authored-by: NLi Fuchen <lfchener@outlook.com>
Co-authored-by: NYibing Liu <liuyibing01@baidu.com>
Co-authored-by: NGuo Sheng <whucsgs@163.com>
Co-authored-by: Nruri <shipeng1108@163.com>
Co-authored-by: NChen Weihang <sunny_cwh@163.com>

This reverts commit adcdb7b8.

* Modify the formula of lstm

* Update README.cn.md

* Update README.cn.md

* Unify 1.6 api in 06.understand_sentiment

* fix a bug in README.md

* Update README.cn.md

* Modify the formula of lstm

Fixed issue with train_dyn_rnn.py training report nan

Modified the bug in understand_sentiment
icafe : http://newicafe.baidu.com/issue/76263402/show?cid=5&spaceId=23205&from=email&noticeStatistics=23969354

* modify understand_sentiment doc

* modify link of understand_sentiment doc

* modify png of understand_sentiment

* Update README.cn.md

* Delete formula_rnn_2.png

* Add files via upload

* Delete formula_lstm_2.png

* Add files via upload

add image

* Update README.cn.md

* Update README.cn.md

* Update README.cn.md

* Update README.cn.md

* Delete formula_lstm_1.png

* Delete formula_lstm_2.png

* Add files via upload

* Delete formula_rnn_2.png

* Update README.cn.md

* Update README.cn.md

* Update README.cn.md

* modify doc

* modify doc

test=develop