For rack PCs, with nVidia (or similar) graphics cards (so, 1060s etc),
we use normal 4u rack mounted PCs.
So if you have 4 or 8 of them in a rack it's obviously a huge amount of height space.
In fact, these days (2018) is there a realistic rack PC that is 1u, but can take a normal nVidia card.
Of course, it has to run server-style 24/7 heat wise.