Hogger can walk out of the map just by walking in at certain angles with the walls. He seems to be the only hero you can do it with at the moment, but I’m not entirely sure. I tested for it on the try mode map, and Infernal Shrines Here’s some screenshots to show you how to do it.
https://imgur.com/a/aSlesRA
Here you can see 4 screenshots of just very easily walking out of the map. It works in other spots as well, but this one is very easy.
That’s not a bug, that’s a feature 
. He can go anywhere on screen he wants. 
Seems you can do it on every place where there’s a specific angle. I think every angle of around 135 degrees is possible. It seems his entire hit box is bugged on most maps as well though as you can push half your hitbox through most walls.