input: Write a function to preprocess MNIST dataset images.
  assert preprocess_images(train_images).shape == (60000, 28, 28, 1)
  assert preprocess_images(test_images).shape == (10000, 28, 28, 1)
  assert preprocess_images(train_images).dtype == 'float32'
output: def preprocess_images(images):
      images = images.astype('float32') / 255.0
      images = images.reshape(-1, 28, 28, 1)
      return images