Solved by hugo_thim in post #3 
              
              
                I believe I have found the answer to my question. Since, tensors can only be concatenated when of identical size, I can simply concatenate each tensors of identical sizes in each of the lists in A and B to make 3D tensors. 
For instance, 
A = [torch.stack(x) for x in list(zip(*A))]
B = [torch.stack(…