Use less GPU memory

Update VarDesc design

* add adam op

moment1_out = beta1 * moment1 + (1 − beta1) * grad
moment2_out = beta2 * moment2 + (1 − beta2) * grad * grad
moment1_hat =  moment1_out / (1 - beta1^t)
moment2_hat =  moment2_out / (1 - beta2^t)
param_out = param - learning_rate * moment1_hat / (sqrt(moment2_hat) +
epsilon)

* fix moment 2

* Adding the Adam optimization operator

* Adding more tests for Adam op

remove unused todo in shape_inference.h

* Remove attr_checker from CreateGradOpDescs()

* Fix merge error

* Fix bug in backward_test.cc

Complete `Operator` of Python API

fix computation graph error

Fix InferShapeContext in decayed_adagrad_op

refine conv2d naive function

* Implementing the DecayedAdagrad optimizer step operator

* implementing DecayedAdagrad operator

* remove file

* small fix

Feature/fix ci

* add cudnn_conv_op

* WIP

* update

* update

* fix grad check

* use platform::memory

* add support group for cudnn

* update

* follow comments

* fix onlycpu build

* update cuda define

* follow comments

* follow comments

* merge with updates

* fix compile error

* follow comments

* follow comments

ImportError: /usr/local/lib/python2.7/dist-packages/py_paddle/_swig_paddle.so: cannot allocate memory in static TLS block

* Adding thresholded_relu op
* Adding test for thresholded relu op

* init

* unify CopyFrom interface

* fix gpu build error

* fix bug in tensor_py.h

* refine code comments and add TODO list

* fix conflicts in FeedOp and FetchOp