Add sequence labeling model for Fluid.

[lcy-seso]

Add sequence labeling model which can be tasks like NER or SRL in NLP to https://github.com/PaddlePaddle/models/tree/develop/fluid .

Can take this model https://github.com/PaddlePaddle/models/tree/develop/sequence_tagging_for_ner as a reference, or any other state-of-art model.

[guru4elephant]

get

[lcy-seso]

Thank you for taking this work.