GPU Support

[willtebbutt]

Ensuring that Stheno supports all of its functionality on the GPU (where appropriate) is quite important moving forwards. This will have to be CuArrays.jl-based. To achieve this, we must ensure that:

• All base mean functions and kernels support GPU inputs
• All operations on all compositions are implemented in terms of operations that CuArrays can support.
• logpdf, rand, elbo, etc work correctly with CuArrays.
• Zygote works efficiently on all of the above.

For the most part, I expect this functionality to require writing quite a number of tests, and making tweaks where necessary.

[willtebbutt]

As regards CI, JuliaGPU have provided these instructions.