Fix complex gradients

[antoine-levitt]

Ref #29

[MikeInnes]

Thanks!

Do you think we could generally re-use the rules in DiffRules for this? (see e.g. the real.jl file for example usage).

[antoine-levitt]

I think it's in general true that any function defined in standard libs that is C -> C is holomorphic, and therefore can be differentiated using the same rules as in this PR. The single exception I can see is conj.

Then there are the C->R functions (abs, abs2, hypot, real, imag) that have to be special-cased, and the comparison-based (max, mod, etc.) that aren't defined for complex arguments.

If there's a reliable way of predicting whether a function is C->C, then that could be used. Otherwise it'll have to be manual.

[MikeInnes]

I suppose that at the very least, we could just make a list of valid functions and look them up in DiffRules, rather than writing out the gradients again. Then at least adding new definitions would be very easy. Not urgent but just a thought for now.