Forbid offsets

[mcabbott]

This follows LinearAlgebra in explicitly forbidding offset arrays. They don't work anyway, but this replaces a BoundsError with something deliberate:

julia> ForwardDiff.gradient(sum, OffsetArray([1,2,3], 3))  # before
ERROR: BoundsError: attempt to access 3-element OffsetArray(::Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}, 4:6) with eltype Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3} with indices 4:6 at index [1:3]
Stacktrace:
  [1] throw_boundserror(A::OffsetVector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}, Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}}, I::Tuple{UnitRange{Int64}})
    @ Base ./abstractarray.jl:703
  [2] checkbounds
    @ ./abstractarray.jl:668 [inlined]
  [3] view
    @ ./subarray.jl:177 [inlined]
  [4] maybeview
    @ ./views.jl:148 [inlined]
  [5] dotview
    @ ./broadcast.jl:1201 [inlined]
  [6] seed!(duals::OffsetVector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}, Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}}, x::OffsetVector{Int64, Vector{Int64}}, seeds::Tuple{ForwardDiff.Partials{3, Int64}, ForwardDiff.Partials{3, Int64}, ForwardDiff.Partials{3, Int64}})
    @ ForwardDiff ~/.julia/dev/ForwardDiff/src/apiutils.jl:65
  [7] vector_mode_dual_eval!
    @ ~/.julia/dev/ForwardDiff/src/apiutils.jl:36 [inlined]
  [8] vector_mode_gradient(f::typeof(sum), x::OffsetVector{Int64, Vector{Int64}}, cfg::ForwardDiff.GradientConfig{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3, OffsetVector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}, Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}}})
    @ ForwardDiff ~/.julia/dev/ForwardDiff/src/gradient.jl:106
  [9] gradient(f::Function, x::OffsetVector{Int64, Vector{Int64}}, cfg::ForwardDiff.GradientConfig{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3, OffsetVector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}, Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}}}, ::Val{true})
    @ ForwardDiff ~/.julia/dev/ForwardDiff/src/gradient.jl:0
 [10] gradient(f::Function, x::OffsetVector{Int64, Vector{Int64}}, cfg::ForwardDiff.GradientConfig{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3, OffsetVector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}, Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}}}) (repeats 2 times)
    @ ForwardDiff ~/.julia/dev/ForwardDiff/src/gradient.jl:17
 [11] top-level scope
    @ REPL[162]:1

julia> ForwardDiff.gradient(sum, OffsetArray([1,2,3], 3))  # after
ERROR: ArgumentError: offset arrays are not supported but got an array with index other than 1

[andreasnoack]

I think it would be better to put the checks in the functions that assume one-based indexing https://github.com/JuliaDiff/ForwardDiff.jl/blob/78c73afd9a21593daf54f61c7d0db67130cf29e1/src/apiutils.jl#L62-L84 where the error you show is thrown from. Or maybe just change the indexing to be generic?

[mcabbott]

My hope is that eventually general indexing could be supported, and these checks can be removed as soon as there are tests that it works.

I didn't bother to figure out precisely where the failures were, but did check that all the functions to which this error was added do currently fail.