* Adding nesterov momentum to python momentum wrapper
* Fixing optimizer test after merge

* Extract apply_backward_pass to backward.py

Rename apply_backward_pass to append_backward_ops

* Fix CI

* Update design doc

* Adding Adam Python wrapper

* Adding tests for Python Adam wrapper

* Adding the interface for the momentum optimizer
* Adding a comment about accumulators

* init parameter base class

* optimize the Comments of optimizer

* basic implimentation of optimizer

* add test_optimizer

* add no_grad_set to interface

* update optimizer.py

* python code can run

* fix some problem

* add sync_with_cpp to Python Program and Block

* sync vars and ops in block from cpp

* optimize code and add some comment

* add more check for sync

* update optimizer with return value of Backward

* rm unused code

* infer shape when create gradient vairiable

* update test_optimizer

* update test_program.py

* update backward test

* follow comment