Opt out of CR.rrule if pullback is not defined #130
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This came up in JuliaGaussianProcesses/KernelFunctions.jl#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 therrulewill 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
rruledefinition. Therefore I'm not sure how useful this PR is currently (and I'm not sure how to test it properly...).