Fix calculation of current item number in optimization_tutorial.py (#2743)

Current item calculation gave the wrong number when dataset doesn't split to batches equally.

Co-authored-by: Svetlana Karslioglu <[email protected]>

Author: anttikup

beginner_source/basics/optimization_tutorial.py

@@ -163,7 +163,7 @@ def train_loop(dataloader, model, loss_fn, optimizer):
         optimizer.zero_grad()
 
         if batch % 100 == 0:
-            loss, current = loss.item(), (batch + 1) * len(X)
+            loss, current = loss.item(), batch * batch_size + len(X)
             print(f"loss: {loss:>7f}  [{current:>5d}/{size:>5d}]")