[Feature] Add fuse vars op handle

rewrite unittest of trt_activation_op

Add unittests to check channelwise add

Add reverse op

install libgtk2.0-dev in latest images

SSA Graph Builder Factory

Extract method from tensor_impl.h to tensor.cc

Fix formula display error

fix cudnn version issue

Fix abort issue in cpu multi-threads

add python-opencv in paddlepaddle/paddle:lastest images

* Enable assertions in CUDA.

* Fix PADDLE_ASSERT.

Merge remote-tracking branch 'ups/develop' into multithreads

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

small clean up and document pointer ownership.

disable failed tests

Try to speed up parallel executor