Dear K. Frank, Thank you for your prompt and detailed answer. It is very clear now.
Can I then transform logits to probabilities by a new network model (code below)
class Sig(nn.Module):
    def __init__(self):
        super(Sig, self).__init__()
    
        self.activation = nn.Sigmoid()      
    def forward(self, x):
        h = self.activation(x) 
        return h

#create a network 
sigmoid = Sig()

and if I may cal this mini model within the training loop to calculate accuracy on the validation set?
Best regards,
Alice