* fix compile error in CUDA10 test=develop

* remove double in pad2d test=develop

* add register op_data_type test=develop

* fix register bug in isfinite op test=develop

* rm int int64_t in pad2d gradKernel  test=develop

* Make pad2d support for variable paddings.
test=develop

* Rename get_paddings and add inline modifier.
test=develop

* Fix comments.

* Add pad2d op.

* Add unitest and python api.

* Fix cuda op kernel.

* Fix python api.

* Fix python api.

* Update API.spec.

* Fix python api