add examples of drake <=> pytorch #447

I should add some examples here of e.g. adding a pytorch model as a cost function to a MathematicalProgram, or using a pytorch model as a system.

We've accumulated examples in many places of the years, e.g.:

• https://github.com/DexaiRobotics/drake-torch
• https://github.com/DAIRLab/drake-pytorch
• https://github.com/robert-verkuil/drake-pytorch

cc @shaoyuancc , @agarwal-abhinav