Support opting out of rules

We provide two ways to know that a rule has been opted out of.

rrule / frule returns nothing

@opt_out defines a frule or rrule matching the signature that returns nothing.

If you are in a position to generate code, in response to values returned by function calls then you can do something like:

res = rrule(f, xs)
if res === nothing
    y, pullback = perform_ad_via_decomposition(r, xs)  # do AD without hitting the rrule
    y, pullback = res

The Julia compiler will specialize based on inferring the return type of rrule, and so can remove that branch.

no_rrule / no_frule has a method

@opt_out also defines a method for ChainRulesCore.no_frule or ChainRulesCore.no_rrule. The body of this method doesn't matter, what matters is that it is a method-table. A simple thing you can do with this is not support opting out. To do this, filter all methods from the rrule/frule method table that also occur in the no_frule/no_rrule table. This will thus avoid ever hitting an rrule/frule that returns nothing (and thus prevents your library from erroring). This is easily done, though it does mean ignoring the user's stated desire to opt out of the rule.

More complex you can use this to generate code that triggers your AD. If for a given signature there is a more specific method in the no_rrule/no_frule method-table, than the one that would be hit from the rrule/frule table (Excluding the one that exactly matches which will return nothing) then you know that the rule should not be used. You can, likely by looking at the primal method table, workout which method you would have it if the rule had not been defined, and then invoke it.