Hi, for the first question, which loss function do you use?
For the second question:
For binary classification, the logistic function (a sigmoid) and softmax will perform equally well, but the logistic function is mathematically simpler and hence the natural choice. When you have more than two classes, you can’t use a scalar function like the logistic function as you need more than one output to know the probabilities for all the classes, hence you use softmax.