* add comments of member <with_grad_semantic>

* polish code

* polish code comments

* add <with_grad_semantic> in OpInputInfo to represent whether an input of OP has grad semantics

* add support for check OP's input grad semantic by comparing fwd_op inputs and bwd_op outputs

* add pybind interface to support checking OP's inputs grad semantic in python-level

* add test

* fix bugs

* fix bugs in op_gen

* fix bugs in op_gen

* add test for multiply_op

* fix bugs in codestype

* fix bugs in codestype