perplexity : add Hellaswag calculation (#2389)
* common.h : add hellaswag / remove perplexity-lines
* common.cpp : add hellaswag / remove perplexity-lines
* perplexity.cpp : add hellswag scores / remove perplexity-lines
* perplexity.cpp : clean up
* common.h : change default param value
* common.cpp : Change default param
* perplexity.cpp : alter wording
* common.h : alter wording
* common.cpp : alter wording
