[misc] Fix training function with distillation (#396)

Author: peterjc123

tinynn/util/cifar10.py

@@ -237,6 +237,7 @@ def _calc_loss(label, label_teacher):
 
         avg_data_time.update(time.time() - batch_end)
         image = image.to(device=context.device)
+        context.optimizer.zero_grad()
 
         if context.grad_scaler:
             with autocast():