use py3 next syntax

Author: ptrblck

examples/int8/training/vgg16/main.py

@@ -93,7 +93,7 @@ def main():
     model = model.cuda()
 
     data = iter(training_dataloader)
-    images, _ = data.next()
+    images, _ = next(data)
 
     writer.add_graph(model, images.cuda())
     writer.close()