MethodError if configured rrule is ambiguous

[oxinabox]

This resolves #1342
which is a special case of ambiguity where the ambiguity is for a configured rule and it's configured opt-out.
But it applies without an opt-out also.

This in particular implements Sol0
Which is to give an useful MethodError rather than a confusing error about no field, as follows:

Before

julia> pullback(h_ambig, 1, 2)
ERROR: type Nothing has no field method
Stacktrace:
 [1] getproperty(x::Nothing, f::Symbol)
   @ Base ./Base.jl:33
 [2] has_chain_rrule(T::Type)
   @ Zygote ~/.julia/packages/Zygote/AS0Go/src/compiler/chainrules.jl:21
 [3] #s2941#1099
   @ ~/.julia/packages/Zygote/AS0Go/src/compiler/interface2.jl:20 [inlined]
 [4] var"#s2941#1099"(::Any, ctx::Any, f::Any, args::Any)
   @ Zygote ./none:0
 [5] (::Core.GeneratedFunctionStub)(::Any, ::Vararg{Any, N} where N)
   @ Core ./boot.jl:571
 [6] pullback(::Function, ::Zygote.Context{false}, ::Int64, ::Vararg{Int64, N} where N)
   @ Zygote ~/.julia/packages/Zygote/AS0Go/src/compiler/interface.jl:44
 [7] pullback(::Function, ::Int64, ::Int64)
   @ Zygote ~/.julia/packages/Zygote/AS0Go/src/compiler/interface.jl:42
 [8] top-level scope
   @ REPL[24]:1

Now:

julia> pullback(h_ambig, 1, 2)
ERROR: MethodError: rrule(::ZygoteRuleConfig{Zygote.Context{false}}, ::typeof(h_ambig), ::Int64, ::Int64) is ambiguous. Candidates:
  rrule(::RuleConfig, ::typeof(h_ambig), x::Int64, y::Int64) in Main at /home/oxinabox/JuliaEnvs/Zygote.jl/test/chainrules.jl:291
  rrule(::ZygoteRuleConfig, ::typeof(h_ambig), x, y) in Main at /home/oxinabox/JuliaEnvs/Zygote.jl/test/chainrules.jl:290
Possible fix, define
  rrule(::ZygoteRuleConfig, ::typeof(h_ambig), ::Int64, ::Int64)
Stacktrace:
 [1] chain_rrule
   @ ~/JuliaEnvs/Zygote.jl/src/compiler/chainrules.jl:223 [inlined]
 [2] macro expansion
   @ ~/JuliaEnvs/Zygote.jl/src/compiler/interface2.jl:0 [inlined]
 [3] _pullback(::Zygote.Context{false}, ::typeof(h_ambig), ::Int64, ::Int64)
   @ Zygote ~/JuliaEnvs/Zygote.jl/src/compiler/interface2.jl:9
 [4] pullback(::Function, ::Zygote.Context{false}, ::Int64, ::Vararg{Int64})
   @ Zygote ~/JuliaEnvs/Zygote.jl/src/compiler/interface.jl:44
 [5] pullback(::Function, ::Int64, ::Int64)
   @ Zygote ~/JuliaEnvs/Zygote.jl/src/compiler/interface.jl:42
 [6] top-level scope
   @ ~/JuliaEnvs/Zygote.jl/test/chainrules.jl:293

This behavior is consistent with our behavour for non-configured rules as implemented in #1342

Sol1 (to just ignore rules) would be easily implemented for both configured and unconfigured cases by changing the final if to is_ambig || match(...) rather than `!is_ambig && match(...)
but should be a separate PR.

PR Checklist

• Tests are added

[ToucheSir]

LGTM, merge when ready :)