You are totally right, and it was 100% my fault for not reading the error message right, I though I got some way different tensor, but actually it was the right one only with the fact, that I resized the CIFAR images for the GAN to 64x64 thus resulting a [100 x 16384] vector and my CNN was expecting 4096 (for 32x32).
So a quick change to TensorDataset(F.interpolate(gen_imgs, 32), torch.from_numpy(np.array([0] * 100))) and everything was working.
So thanks for pointing that out!