Modify PyReader doc in python/paddle/fluid/layers/io.py

* expose rank_loss python api

Speedup reduce sum grad op

Skip reader when save

Speedup reduce sum grad op

Some enhancement on readers

disable warning

* 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.

Fix .sh file

* 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

Change and polish readers

* Fix Reduce and Gather

* Fix unit test

Fix deserialize bug

add option split mkl gemm