* 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

* Also simplify pybind implementation by using OperatorBase as holder
  type.

* add NDEBUG switch to PADDLE_ENFORCE

* Free will be added soon