Add Statistics functions

As mentioned by user ogiod on discourse [here](https://discourse.julialang.org/t/statistics-functions-on-gpu/42245) we still lack some statistical functions.

We still need to implement

- [x] `cor`
- [x] `cov`
- [ ] `median` : Scalar operation when dims is mentioned
- [ ] `middle` : Implement extrema function
- [ ] `quantile` : Probably need sort for this

Base's Statistics can be found here : [https://github.com/JuliaLang/Statistics.jl/blob/master/src/Statistics.jl](https://github.com/JuliaLang/Statistics.jl/blob/master/src/Statistics.jl)

CUDA.jl's Statistics can be found here : [https://github.com/JuliaGPU/CUDA.jl/blob/master/src/statistics.jl](https://github.com/JuliaGPU/CUDA.jl/blob/master/src/statistics.jl)