Lookup table support selected rows as parameter

* update index_en.rst

* fix comments

* fix uppercase

temporarily disable ncclBcastOp test, it fails randomly

Add paddle.v2.reader,dataset back for backward compatibility

Merge branch 'develop' of https://github.com/PaddlePaddle/Paddle into lookup_table_support_SelectedRows_as_parameter

disable test_recv_op

Add local cache of double buffer reader

fix mac compile

Improve prefetch on server

fix single pserver single trainer error

Enhance reshape

* "polish code"

* "fix ci"

* "fix ci"

* "done"

Add prefetch interface on server side

* translate api standard

* Update api_doc_std_en.md

* fix typo