fix fluid_benchmark.py

* add flatten op

* enable dist se resnext

* small batch size

* refine regularization for selected_rows

* clean lookup_table

* refine rpc_server_test

* temporally disable rpc_server_test

* fix rpc_server_test

* add unit test

* update test_pe_mnist threshold

* clean code

* Hiden some LoDTensor ralatted ops' Python wrapper.

* hide variable API

* edit API.spec

* expose rank_loss python api

* get diff for all changes in current branch

* diff with upstream/develop

* let dist test serial

1. Make the feeding thread of py_reader a daemon thread.
2. Update buffer_reader's destructor, fixing a bug.
3. Make pyreader demo script supporting CPU environment.

* Fix serious bug in nesterov momentum optimizer.

* wip add se-resnext test for 2x2 and parallel exe

* testing

* finish dist unitest 2x2

* update python interp

* fix gpu number

* make this test can run in parallel

* Fix Reduce and Gather

* Fix unit test

fix AUC op
optimize it's test

* Added beam search API to fluid/contrib

* Change as review comments

* API.spec change

* Doc updated