Solved by BTNug in post #2 
              
              
                Hi @Patrice, 
*Rule1: If you want to concatenate the layer, it has to have only 1 dimension that is different from the other (i.e. NxDiff1xHxW and NxDiff2xHxW or NxCxDiff1xW and NxCxDiff2xW, etc) 
in your case, what you are trying to do: 
suppose our input x: 1x3x28x28 
what you are doing is: 

you …