use int64 replace int
 uename
 add unittest for feature_encoder
 add unittest
 add bigram feature extract
add extract features
 rename cost to unigram_cost and add bigram_cost
 fixbug : wrong name
 add unittest
 fix a bug
 try add bigram in calc-arena
 passed, add function in crf_model unigram_index, and  bigram_index
 grad with bigram
 .