Added contextualized representation to ner model

[xtang27]

Added language model optional parameters to the training script. If LMs are given, it will add contextualized representation as part of the input feature. It also needs to have model_word_ada and model_seq modules from LD-Net repository in its directory.

Link to project report: https://drive.google.com/file/d/1FCYnQ2ZaPQJA0dckkw6gm1-om9KNPrVP/view Error reduction rate with contextualized representation: ~16.5%