Simplify frule for Base.tail

[mcabbott]

Correction to #643 to avoid this:

julia> frule((NoTangent(), NoTangent()), Base.tail, (3,4))
ERROR: MethodError: no method matching length(::NoTangent)

Closest candidates are:
  length(::Union{Base.KeySet, Base.ValueIterator})
   @ Base abstractdict.jl:58
  length(::Union{NNlib.BatchedAdjoint{T, S}, NNlib.BatchedTranspose{T, S}} where {T, S})
   @ NNlib ~/.julia/packages/NNlib/uDZz6/src/batched/batchedadjtrans.jl:58
  length(::Union{DataStructures.OrderedRobinDict, DataStructures.RobinDict})
   @ DataStructures ~/.julia/packages/DataStructures/59MD0/src/ordered_robin_dict.jl:86
  ...

Stacktrace:
 [1] _similar_shape(itr::NoTangent, #unused#::Base.HasLength)
   @ Base ./array.jl:660
 [2] _collect(cont::UnitRange{Int64}, itr::NoTangent, #unused#::Base.HasEltype, isz::Base.HasLength)
   @ Base ./array.jl:715
 [3] collect(itr::NoTangent)
   @ Base ./array.jl:709
 [4] _totuple(::Type{Tuple}, ::NoTangent)
   @ Base ./tuple.jl:401
 [5] Tuple(itr::NoTangent)
   @ Base ./tuple.jl:369
 [6] frule(::Tuple{NoTangent, NoTangent}, #unused#::typeof(Base.tail), x::Tuple{Int64, Int64})
   @ ChainRules ~/.julia/packages/ChainRules/BbzFc/src/rulesets/Base/indexing.jl:104

which shows up in Diffractor's tests, e.g. here:
https://github.com/JuliaDiff/Diffractor.jl/runs/7414875951?check_suite_focus=true#step:6:259

Relies on JuliaDiff/ChainRulesCore.jl#567

[mcabbott]

BTW, why doesn't frule(::Tuple{NoTangent, NoTangent}, f, x) just return f(x), NoTangent() always? E.g. by adding dispatch at https://github.com/JuliaDiff/ChainRulesCore.jl/blob/main/src/rules.jl#L64

[mzgubic]

BTW, why doesn't frule(::Tuple{NoTangent, NoTangent}, f, x) just return f(x), NoTangent() always? E.g. by adding dispatch at https://github.com/JuliaDiff/ChainRulesCore.jl/blob/main/src/rules.jl#L64

Didn't think about this very deeply but it does seem fine. In fairness frules have not been used too much in the wild I think, so may just be a few low hanging fruit lying around. Could it work for Tuple{Vararg{NoTangent}} as well?