Compute `Enzyme` gradients with CUDA backend

[SouthEndMusic]

Computing gradients of the spline grid evaluation with respect to the control points with the CUDA backend causes a crash (test_EnzymeExt.jl). Investigate whether this crash can be prevented.

This is a follow up of #62.

[SouthEndMusic]

It's worth checking whether removing @Const in the relevant kernel signatures resolves the problem (since I think in backpropagation it is different which arrays are constant and which aren't).