Thank you very much my mistake was in the input shape and input size 
I have an other stupid questions :
one of my convolution take as input, two outputs from two past Relus. Those inputs must be summed . I did the stupid first thing that came up to my mind which is a simple addition which obviously gives me the error:

RuntimeError: The size of tensor a (60) must match the size of tensor b (56) at non-singleton dimension 2

Any idea ?
thanks a lot for your link I am looking at it to apply it to my code 
Justin