Hi, Excellent tutorials! But I have a question. Form tutorial 13 and on you change the place where the zero_grad method is called and I do not get why?
Before 13 was:

loss = criterion(outputs, labels)
loss.backward()
optimizer.step()
optimizer.zero_grad()

After 13:

loss = criterion(outputs, labels)
optimizer.zero_grad() # Here is the change
loss.backward()
optimizer.step()

Now I am wondering if you set to zero the gradients, then, how the optimizer could update the parameters without any information about the gradient?