Opt out of CR.rrule if pullback is not defined

[devmotion]

This came up in https://github.com/JuliaGaussianProcesses/KernelFunctions.jl/pull/386:

Sometimes you might want to use grad=false, e.g., if there is neither a rule in DiffRule nor do you want to use ForwardDiff. However, in this case the rrule will always error, even if your CR-compatible AD system could differentiate through your function just fine. This PR adds an opt-out definition if no pullback is defined so that the AD system can take over.

Caveat: I don't think it's very useful in practice currently since (almost?) no CR-compatible AD system supports mutation. E.g., Zygote will just throw a mutation error instead of the custom error in the rrule definition. Therefore I'm not sure how useful this PR is currently (and I'm not sure how to test it properly...).