* Fixing unbufered test to a generic test

* Update channel_test.cc

* splitting over functions

* add type

* fix *

* Adding more details to cluster_train

* Update copyright for notest_dist_image_classification

* Fixed copyright

* Updating the copyright year and content

Fix for program crash when destructor is called before channel close with blocked readers/writers (#8197)

* Fix destructor crash and add unit tests

* Fix typo in unit test

* Reword comments

* Make close channel a generic test

* Refactoring unit tests

* Fix method name

* Adding panic logic and test case

* Change panic behavior to boolean instead of exception

* Adding atomic

* Switch to boolean

* Fix spacing

* Add to close method

* Add test case to return zero on a closed channel

* Rename method

* Fix test

* ReceiveFromBufferedChannelReturnResidualValuesTest

* Adding the variable and case for unbuffered channel

* Fix review comments

* Fix format

* Remove a zero-value comparison

Change the behaviour of thread pool. Thread pool will ignore the
thrown exception implicitly. It is hard to debug.
Corrently, ThreadPool::Run will invoke `LOG(FATAL)` if an exception
thrown.

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

* refine buffer channel

*  refine Receive and Send

* follow comments

* Adding more receivers less senders

* Added more receivers less senders

* Added more send

* Updated comment

* Fixed code style

* Fixing review comments