There is a heat sink on the 2012, step 14 and 15 I can't see why it would be any different on the 2013, but I can't find a teardown of the 2013 15" with a dGPU in it.
That's what I linked to, you can see the heat sink and dGPU in step 14 and 15 (they even talk about it), the heat sink and dGPU are a quarter inch to the left of the screwdriver in the first picture for step 14. It's also marked in the step 15 picture.