Why is the dispatcher the way it is today? How did evolve over time, and what constraints got added so that it is the kind of complicated piece it is today?

Further reading:

• How the dispatcher actually works http://blog.ezyang.com/.../lets-talk-about-the-pytorch.../
• Zachary DeVito's original version of ATen, before it got merged back into PyTorch mainline https://github.com/zdevito/ATen
• The multiple dispatch patch: https://github.com/pytorch/pytorch/pull/25653