Solved by ptrblck in post #4 
              
              
                Thanks for the code. 
Your z tensor is on the CPU, while the discriminator is on the GPU. 
While I can reproduce this error in 1.6.0, it seems to have been fixed to a proper error message in the latest nightly binary (tested with 1.7.0.dev20200829): 

RuntimeError: Tensor for argument #2 ‘mat1’ is o…