Add token unique method described in the paper

To distinguish same words in same document as different token, add the method `normalize_words` to assign a number to the same words.
for example "as soon as possible" wil become "as@#0 soon@#0 as@#1 possible@#0", just for speration for the index and the origin word itself.  No special meaning, we could also remove the seperation, just add number of index :)