Add a new type `Reader` for `VarDesc`, which can holds more than one
LoDTensor.

simplify shape inference code

Refine elementwise_op

Add v2 dist benchmark vgg

 Add multi-class non-maximum suppression operator.

Reclassify and change of V2 API documentation structure

add mine_hard_examples operator

add box coder op

Allow uers to specify the name of moving mean and variance in batch_norm interface.

add switch op design

Adding unit tests for checking that closing unbuffered channel closes all blocked senders and receivers (#8067)

* Adding unit tests for checking that closing unbuffered channel closes all blocked senders and receivers

* Fixing sleep time

init debuger.

Make NMT as normal python unittests

Add softmax into Python API.

Make recognize digits as a normal python unittest

* init

* add delete operator

* debug

* add wait

* clean code

* fix bug

* fix bug

* refine code

* remove unused code

Some optimization of CNN model computation.

* Add unite test with less receivers, more senders

* Fixed the check

* Adding an initial implementation for the unbuffered channel

* Including atomic header

* update comment

* Adding the closed attribute

* Add comments

* Updated locking mechanism

* Add simple unbuffered test

* Enhance unit test and fix bug

* Add details

Fix document errors.

* "clean code"

* "clean code"