* 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

Following the design
* https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/python_api.md

Just written `Program`, `Block` and unittest of program.