Update nn_tutorial.py (#2368)

* Update nn_tutorial.py

Fix to #1303 "add pyplot.show() in beginner tutorial."

Comments to issue suggested manually commenting out pyplot.show for users not using colab. 

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Co-authored-by: Svetlana Karslioglu <svekars@fb.com>

Author: frasertajima

beginner_source/nn_tutorial.py

@@ -75,6 +75,11 @@
 import numpy as np
 
 pyplot.imshow(x_train[0].reshape((28, 28)), cmap="gray")
+# ``pyplot.show()`` only if not on Colab
+try:
+    import google.colab
+except ImportError:
+    pyplot.show()
 print(x_train.shape)
 
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