fix: MNIST minor bug (#5075)

Co-authored-by: Roger Shieh <[email protected]>

Author: Shumpei-Kikuta

docs/source/introduction_guide.rst

@@ -601,8 +601,8 @@ In this method we do all the preparation we need to do once (instead of on every
         def setup(self, stage):
             # transform
             transform=transforms.Compose([transforms.ToTensor()])
-            MNIST(os.getcwd(), train=True, download=False, transform=transform)
-            MNIST(os.getcwd(), train=False, download=False, transform=transform)
+            mnist_train = MNIST(os.getcwd(), train=True, download=False, transform=transform)
+            mnist_test = MNIST(os.getcwd(), train=False, download=False, transform=transform)
 
             # train/val split
             mnist_train, mnist_val = random_split(mnist_train, [55000, 5000])