Reduce one level of inheritence.

add doc

* 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

* Fix tensorcopy bug

* follow comment

* Refine TensorCopy

* 1. Create buddy allocator in each places before NcclBcast the variables
2. Check the memory usage of ALL gpus rather than the first one

* 1. Make NCCLGroupGuard guards only the ncclBcast part, which avoid ncclGroupEnd blocking the exception throwing
2. NOTE the usage of NCCLGroupGuard

* Remove the memory usage check of gpus

* Fix code style

replace use_event with use_cuda, because use_event means the program running with CUDA, so use_cuda maybe more intuitive.

* Use Builder Chain to decorate new builders. It is easy to extend
  builders.
* Make graphviz path as a build strategy, not a FLAGS.