Solved by ptrblck in post #6 
              
              
                It’s a limitation from the cudnn implementation. 
From the cudnn docs: 

fwdMode Input. Specifies inference or training mode (CUDNN_FWD_MODE_INFERENCE and CUDNN_FWD_MODE_TRAINING). In the training mode, additional data is stored in the reserve space buffer. This information is used in the backward p…