Dear @ptrblck
After the exercise with Resnet20 described above, I have run the same scripts with a pure convolutional network (no BN) .
Then, the result is conform of what we expect
that is to say after reloading the network it gives the same outputs for the same inputs.
max abs error:  0.0

So,

we can conclude that the debugging code is working
in the case of the Resnet20 network we should investigate what can produce the difference we see after reloading the model.

Thanks and happy x-mas.