It’s not a trainer or integrated into Go itself if you just use the map
For about a year now modders have been furiously writing scripts and programs that augment themselves to Pokemon Go and add features the developers haven’t implemented or have taken out — like actual creature tracking and the like.
Niantic has banned the use of those apps periodically (sometimes claiming that they slow down their infrastructure and cause crashes), including ones that are full-blown cheat programs that spoof their location so they can access Pokemon or Gyms that they aren’t physically located at. But if you’re so inclined, GymHuntr has been updated to showcase raid locations without any risk to your account.
You just head to the website, put your location in, and you’re good to go. A Google Maps-esque aerial view will show you local gyms and raids, as well as the timer on each so you know if you have enough time to grab lunch before taking down a boss. In my experience it’s fairly accurate, so give it a try.