[Tensorflow] Building RNN Models to Solve Sequential MNIST

In this post, we're going to lay some groundwork for the custom model which will be covered in the next post by familiarizing ourselves with using RNN models in Tensorflow to deal with the sequential MNIST problem. I've put the source code for this post in a notebook hosted on Google Colaboratory, which kindly provides a free GPU runtime for the public to use （I kept getting disconnected to the runtime when running the notebook. So some of the model training was not completed. You can copy the notebook and run it yourself.）: The notebook should have done most of the talking.