Solved by acobobby in post #2 
              
              
                You can proceed like this: 
first = input_tensor.repeat(1, 2, 1)
second = input_tensor.unsqueeze(2)
second = second.repeat(1,1,2,1).view(input_tensor.size(0),-1,input_tensor.size(2))
output_tensor = torch.cat((first,second), dim=2)