Fix MNIST classification example (#19072)

Co-authored-by: r3stl355 <ulmasov@amazon.com>

CONTRIBUTORS.md

@@ -284,6 +284,7 @@ List of Contributors
 * [Connor Goggins](https://github.com/connorgoggins)
 * [Wei Chu](https://github.com/waytrue17)
 * [Joe Evans](https://github.com/josephevans)
+* [Nikolay Ulmasov](https://github.com/r3stl355)
 
 Label Bot
 ---------

example/gluon/mnist/README.md

@@ -21,10 +21,9 @@ This script shows a simple example how to do image classification with Gluon.
 The model is trained on MNIST digits image dataset and the goal is to classify the digits ```0-9```.  The model has the following layout:
 ```
 net = nn.Sequential()
-with net.name_scope():
-    net.add(nn.Dense(128, activation='relu'))
-    net.add(nn.Dense(64, activation='relu'))
-    net.add(nn.Dense(10))
+net.add(nn.Dense(128, activation='relu'))
+net.add(nn.Dense(64, activation='relu'))
+net.add(nn.Dense(10))
 ```
 
 The script provides the following commandline arguments: 

example/gluon/mnist/mnist.py

@@ -48,10 +48,9 @@
 # define network
 
 net = nn.Sequential()
-with net.name_scope():
-    net.add(nn.Dense(128, activation='relu'))
-    net.add(nn.Dense(64, activation='relu'))
-    net.add(nn.Dense(10))
+net.add(nn.Dense(128, activation='relu'))
+net.add(nn.Dense(64, activation='relu'))
+net.add(nn.Dense(10))
 
 # data