As expected, network provider AT&T has not enabled Wi-Fi hotspot functionality on the HTC One. There is a mod available that enables it and the best part is that it works with both rooted and non rooted HTC One phones. The only thing required is CWM recovery.
Download the mod from here and put it on the root directory on your phone. Go into recovery and flash it. It is advisable to wipe cache and dalkvik cache. Reboot your phone and you are done.
Make sure not to abuse this service as it might raise flags. I have been using it for years and have never been flagged.