Errors from AbstractSparseMatrix{Bool}

[mcabbott]

As reported here, these should agree:

julia> using SparseArrays, Zygote

julia> gradient(x -> dot([1,2.0], x, [3,4.0]), diagm([true, false]))
(nothing,)

julia> gradient(x -> dot([1,2.0], x, [3,4.0]), spdiagm([true, false]))
ERROR: MethodError: no method matching zero(::Nothing)
Stacktrace:
  [1] iszero(x::Nothing)
    @ Base ./number.jl:42
...
  [4] map(::typeof(Zygote.wrap_chainrules_output), ::SparseMatrixCSC{ChainRulesCore.NoTangent, Int64})
    @ SparseArrays.HigherOrderFns ~/.julia/dev/julia/usr/share/julia/stdlib/v1.10/SparseArrays/src/higherorderfns.jl:1187 [inlined]
  [5] wrap_chainrules_output(dxs::SparseMatrixCSC{ChainRulesCore.NoTangent, Int64})
    @ Zygote ~/.julia/packages/Zygote/HTsWj/src/compiler/chainrules.jl:127 [inlined]
  [6] wrap_chainrules_output(x::ChainRulesCore.Thunk{ChainRules.var"#1950#1954"{…}})
    @ Zygote ~/.julia/packages/Zygote/HTsWj/src/compiler/chainrules.jl:110 [inlined]
...

Could be fixed at many levels:

• wrap_chainrules_output(::AbstractArray{NoTangent) should give nothing, not call map.
• In rule for dot here, ProjectTo should not make an array of eltype NoTangent.

julia> using ChainRulesCore

julia> ProjectTo(diagm([true, false]))
ProjectTo{NoTangent}()

julia> ProjectTo(spdiagm([true, false]))
ProjectTo{SparseMatrixCSC}(element = ProjectTo{NoTangent}(), axes = (Base.OneTo(2), Base.OneTo(2)), rowval = [1, 2], nzranges = UnitRange{Int64}[1:1, 2:2], colptr = [1, 2, 3])

• In fact, the rule should not compute anything before discarding it. Perhaps it should call ProjectTo{NoTangent}(::Thunk) to discard the thunk immediately.