I was looking to this post (UNet implementation 42 a bit old) where apparently in pytorch there were some issues to implement Unet.  I could not find a real solution for the moment, Saed in one reply wrote only
"For the last set of convolutions, that is 128-> 64 -> 64 -> 1, the activation function should not be used!
The activation function causes the values to vanish!
I just removed the nn.ReLU() modules on top of these convolution layers and now everything works like a charm! "