Fix numerical abs usage in primal-dual residual gap computation#61

ZedongPeng merged 2 commits intoMIT-Lu-Lab:mainMIT-Lu-Lab/cuPDLPx:mainfrom

LucasBoTang:fix/issue-60-residual-gapLucasBoTang/cuPDLPx:fix/issue-60-residual-gapCopy head branch name to clipboard