Fix Index of blitz (#863)

2nd and 4th is identical and miss *Optional: Data Parallelism*.
Add the 5th `.py`,  “Optional: Data Parallelism”

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

Author: H-xie

beginner_source/blitz/README.txt

@@ -13,12 +13,11 @@ Deep Learning with PyTorch: A 60 Minute Blitz
 	Neural Networks
 	https://pytorch.org/tutorials/beginner/blitz/neural_networks_tutorial.html#
 
-4. autograd_tutorial.py
-	Automatic Differentiation 
-	https://pytorch.org/tutorials/beginner/blitz/autograd_tutorial.html
-
-5. cifar10_tutorial.py
+4. cifar10_tutorial.py
 	Training a Classifier
 	https://pytorch.org/tutorials/beginner/blitz/cifar10_tutorial.html
 
+5. data_parallel_tutorial.py 
+	Optional: Data Parallelism
+	https://pytorch.org/tutorials/beginner/blitz/data_parallel_tutorial.html