Reverse mode `jacobian` is wrong for matrix input

[gdalle]

This seems related to the wrong jacobian shape of #1332 but it turns out even a reshape is not enough:

julia> using Enzyme

julia> x = float.(reshape(1:6, 2, 3))
2×3 Matrix{Float64}:
 1.0  3.0  5.0
 2.0  4.0  6.0

julia> f(x) = fill(sum(abs2, x), 6);

julia> jac_forward = Enzyme.jacobian(Enzyme.Forward, f, x)
6×6 Matrix{Float64}:
 2.0  4.0  6.0  8.0  10.0  12.0
 2.0  4.0  6.0  8.0  10.0  12.0
 2.0  4.0  6.0  8.0  10.0  12.0
 2.0  4.0  6.0  8.0  10.0  12.0
 2.0  4.0  6.0  8.0  10.0  12.0
 2.0  4.0  6.0  8.0  10.0  12.0

julia> jac_reverse = reshape(Enzyme.jacobian(Enzyme.Reverse, f, x, Val(6), Val(1)), 6, 6)
6×6 Matrix{Float64}:
  2.0   2.0   2.0   4.0   4.0   4.0
  6.0   6.0   6.0   8.0   8.0   8.0
 10.0  10.0  10.0  12.0  12.0  12.0
  2.0   2.0   2.0   4.0   4.0   4.0
  6.0   6.0   6.0   8.0   8.0   8.0
 10.0  10.0  10.0  12.0  12.0  12.0

[gdalle]

This is now solved, although the shape of the forward Jacobian has changed:

julia> using Enzyme

julia> x = float.(reshape(1:6, 2, 3))
2×3 Matrix{Float64}:
 1.0  3.0  5.0
 2.0  4.0  6.0

julia> f(x) = fill(sum(abs2, x), 6);

julia> jac_forward = Enzyme.jacobian(Enzyme.Forward, f, x)

6×2×3 Array{Float64, 3}:
[:, :, 1] =
 2.0  4.0
 2.0  4.0
 2.0  4.0
 2.0  4.0
 2.0  4.0
 2.0  4.0

[:, :, 2] =
 6.0  8.0
 6.0  8.0
 6.0  8.0
 6.0  8.0
 6.0  8.0
 6.0  8.0

[:, :, 3] =
 10.0  12.0
 10.0  12.0
 10.0  12.0
 10.0  12.0
 10.0  12.0
 10.0  12.0

julia> jac_reverse = Enzyme.jacobian(Enzyme.Reverse, f, x, Val(6), Val(1))
6×2×3 PermutedDimsArray(::Array{Float64, 3}, (3, 1, 2)) with eltype Float64:
[:, :, 1] =
 2.0  4.0
 2.0  4.0
 2.0  4.0
 2.0  4.0
 2.0  4.0
 2.0  4.0

[:, :, 2] =
 6.0  8.0
 6.0  8.0
 6.0  8.0
 6.0  8.0
 6.0  8.0
 6.0  8.0

[:, :, 3] =
 10.0  12.0
 10.0  12.0
 10.0  12.0
 10.0  12.0
 10.0  12.0
 10.0  12.0

julia> reshape(jac_reverse, 6, 6)
6×6 reshape(PermutedDimsArray(::Array{Float64, 3}, (3, 1, 2)), 6, 6) with eltype Float64:
 2.0  4.0  6.0  8.0  10.0  12.0
 2.0  4.0  6.0  8.0  10.0  12.0
 2.0  4.0  6.0  8.0  10.0  12.0
 2.0  4.0  6.0  8.0  10.0  12.0
 2.0  4.0  6.0  8.0  10.0  12.0
 2.0  4.0  6.0  8.0  10.0  12.0

julia> jac_reverse == jac_forward
true