Fix backpropagation order in optimization tutorial

Reorder optimizer.zero_grad(), loss.backward(), and optimizer.step()
to match the recommended best practice documented in the tutorial.

Fixes #3507

Author: patrocinio

beginner_source/basics/optimization_tutorial.py

@@ -158,9 +158,9 @@ def train_loop(dataloader, model, loss_fn, optimizer):
         loss = loss_fn(pred, y)
 
         # Backpropagation
+        optimizer.zero_grad()
         loss.backward()
         optimizer.step()
-        optimizer.zero_grad()
 
         if batch % 100 == 0:
             loss, current = loss.item(), batch * batch_size + len(X)