Register custom adjoints

[avik-pal]

Useful in general but pretty much needed for things like https://discourse.julialang.org/t/how-to-compute-tanhexp-telu-function-accurately/124464/4 where applying the direct chain rule is numerically unstable

[wsmoses]

Technically the requirement is a bit deeper where we need to enable EnzymeMLIR to support custom derivatives. Probably the right approach is for code with a custom derivative, we register and emit a new op with a definition (like a special call op), whose derivative is something we register