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 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

* remove block api

* remove clone_variable

* hide block inner apis

* update

* fix tests

* fix auc

* update

* update

* fix compile

* fix param name

* add doc string

* fix test

* Add learning rate decay test

* fix test name

* doesn't share @LR_DECAY_COUNTER@

* refine multi-thread CPU Parallel exe

* refine multi thread CPU Parallel exe

* Refine CPU version for ParallelExecutor

* add share_parameter_between_cards_

* Fix ParallelExecutor bug

* Fix unit test

* Fix parameter opt balance

* Fix with opti (param->grad)

* Add grad to op var

* Remove shard_param_between_cards