guide for adding new instructions?

[GallagherCommaJack]

I was trying to implement a fused abs_angle kernel for complex tensors, and needed to use inverse trigonometric ops (specifically atan), and noticed that the available math instructions are very sparse. I then tried to add an atan instruction, expecting to open a PR, but after a couple hours of chasing build errors I couldn't figure out how to add it - some documentation would be much appreciated.