Update MNIST DCGAN notebook

[AAnirudh07]

Hello @Yangyangii! Firstly, thank you for creating this super comprehensive notebook on GANs! I tried executing the DCGAN notebook for the MNIST dataset and noted that the outputs weren't getting saved to the samples directory since it didn't exist. I added the following line of code to fix this: if not os.path.exists('samples'): os.makedirs('samples')