Update autogradyt_tutorial.py (#2247)

Fix #2219

Author: Svetlana Karslioglu

beginner_source/introyt/autogradyt_tutorial.py

@@ -334,7 +334,7 @@ def forward(self, x):
 
 print(model.layer2.weight.grad[0][0:10])
 
-optimizer.zero_grad()
+optimizer.zero_grad(set_to_none=False)
 
 print(model.layer2.weight.grad[0][0:10])