My mistake, I double checked my code today and I realized I used the .cpu() routine. According to the documentation at https://pytorch.org/docs/stable/tensors.html#torch.Tensor.cpu 1:
Returns a copy of this object in CPU memory. If this object is already in CPU memory and on the correct device, then no copy is performed and the original object is returned.
So I was just normalizing twice my inputs, since the second call returned me the already unnormalized image.
A solution to work on a copy of the tensor is to use torch.Tensor.clone().