It can be done in software now. But this is either an SMF tool or you have to PLAY the melody to give the correct pitch for the autotune to work, even if built into the arranger. This is never going to work without YOU providing the correct notes in realtime in arranger mode.

The trick is, if you use auto tune as a general 'tweak-up' mode, you can't sing more than a quarter-tone sharp or flat or the corrected pitch will go wrong (and especially if you sit close to that quarter tone out of tune,. you get the famous Auto-Tune 'wobble/trill'). For this to be as effective, you need to input the EXACT notes and the autotune will put you to that note no matter WHAT you sing...