Script TF-DO MNIST run.

[cconvey]

This Issue is a baby-step towards gathering the speed- and accuracy-data characterizing the TF DO code on MNIST.

For this Issue:

• Develop a Bash or Python script that simply runs MNIST on the TF-DO code. The script must run successfully on our test machine, assuming that a human has already performed appropriate setup steps.

• Have @yxlao verify that the MNIST script is sufficiently similar to the version we'll use with with our XLA-based code.

• Create documentation describing all human steps needed to run the TF DO script in a clean test environment. (Validation of this step is on a best-effort basis.)