* add target to Backward, generate var in block when call backward

* modify backward_test

* fix executor_test

* set var desc default type to LOD_TENSOR

* update backward_test

* insert loss in the top level of backward

* create grad vars for all blocks in current program

* optimize code

* update test_program.py

* only create var for newly create blocks when backward

* Add debug string for Python ProtoBuf

and Rename `Sync` to `Flush`

* Add check of ProtoBuf initialization

* Add grad_name_map

* Fix bug

* Fix bug

* Follow comments

add selected rows

* Update VarDesc from design doc

* Fix GCC compile

* Fix unittest

Fix pybind op register nokernel

Add gru_unit_op

* Adding Hard Sigmoid Activation

* Adding a comment for slope to be only positive

* Fixing grammatical mistake in comment

Use MinSizeRel compile third_party library when build for mobile infe…

* Add no_grad_vars for grad_op_maker

* Add unittest

* Fix unittest

* Fix unittest

* Follow comment

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