Change get_lr() call to get_last_lr() as per warning (pytorch#1099)

Co-authored-by: holly1238 <77758406+holly1238@users.noreply.github.com>

beginner_source/transformer_tutorial.py

@@ -281,7 +281,7 @@ def train():
             print('| epoch {:3d} | {:5d}/{:5d} batches | '
                   'lr {:02.2f} | ms/batch {:5.2f} | '
                   'loss {:5.2f} | ppl {:8.2f}'.format(
-                    epoch, batch, len(train_data) // bptt, scheduler.get_lr()[0],
+                    epoch, batch, len(train_data) // bptt, scheduler.get_last_lr()[0],
                     elapsed * 1000 / log_interval,
                     cur_loss, math.exp(cur_loss)))
             total_loss = 0