Compute ngram overlap score of source and example as sentence_bleu score
from NLTK package.
Use sentence_bleu with method1 smoothing function and auto reweighting.
Return float value between 0.0 and 1.0 inclusive.
https://www.nltk.org/_modules/nltk/translate/bleu_score.html https://aclanthology.org/P02-1040.pdf
