Can't find co-design demo

[ehannigan]

I've been doing some reading on differentiable physics simulators (I bothered you over on DiffTaiChi as well). I read through the ChainQueen paper. Section V. c. shows a cool co-optimization of a controller along with the Youngs modulus for the multi link deformable arm. I have tried to find that example in this code base, but I am having trouble. I am guessing that it is the arm_3d.py demo because both Young's modulus and the controller are trainable variables, but I cannot seem to get arm_3d to run (it starts an ipython session instead). Am I at least barking up the right tree? I'm happy to add in documentation to the demos that I get running if you let me know which ones I should be looking at.