hessians/jacobians trigger scalar indexing for CuArrays

[marius311]

Zygote v0.6.51 MWE:

using Zygote, CUDA
CUDA.allowscalar(false)
Zygote.hessian(x -> norm(sin.(x)), cu(rand(2)))

I think (but not tested) that this would be fixed by #1270, perhaps this can be added as a test there or wherever this is fixed.