* add padding in linear_chain_crf op

* modify API.spec

* add linear_chain_crf_op.cc and linear_chain_crf_op.h

* remove useless unit test , test=develop

* modify API.spec, test=develop

* remove some blanks in nn.py , test=develop

* fix some bugs on nn.py and API.spec ,test=develop

* fix nn.py, test=develop

* fix API.spec ,test=develop

* fix bug of CI test in test_linear_chain_crf_op.py

* fix bug of CI test in test_linear_chain_crf_op.py, test=develop

* remove paddle_enforce, test=develop

* remove paddle_enforce, test=develop

* remove paddle_enforce, test=develop

* remove paddle_enforce, test=develop

* remove paddle_enforce, test=develop

* remove paddle_enforce, test=develop

* modify nn.py, test=develop

* fix API.spec, test=develop

* fix unittest bug, test=develop