Complex number wrapper

NCCL does not support complex numbers directly and does not plan to ([see issue](https://github.com/NVIDIA/nccl/issues/539)). Are we willing to add a wrapper to NCCL.jl to make using complex numbers more convienient? Alternatively, the wrapper could be put in a higher level package (ex. Lux.jl, [see issue](https://github.com/LuxDL/Lux.jl/issues/747)). I am happy to start working on this but would like some feedback if possible. My primary motivation is using neural networks with complex valued weights and this feature would greatly simplify things.