Bug in `dense_dims` for reshaped views

[ranocha]

Reported in https://github.com/JuliaArrays/ArrayInterface.jl/pull/156#issuecomment-851002133:

julia> using ArrayInterface

julia> u_base = randn(10, 10); u_view = view(u_base, 3, :); u_reshaped_view = reshape(u_view, 1, size(u_base, 2));

julia> ArrayInterface.dense_dims(u_reshaped_view)

As confirmed by @chriselrod in https://github.com/JuliaArrays/ArrayInterface.jl/pull/156#issuecomment-851293134, this is a bug:

The correct answer for u_reshaped_view would be (False(),False()).

[Tokazama]

resolved with #161

[ranocha]

I fixed it only for the case of reshaped versions of abstract vectors. It still fails for reshaped views of higher-dimensional arrays, e.g.,

(@v1.7) pkg> activate --temp
  Activating new project at `/tmp/jl_4vH1Gz`

(jl_4vH1Gz) pkg> add ArrayInterface
    Updating registry at `~/.julia/registries/General.toml`
   Resolving package versions...
    Updating `/tmp/jl_4vH1Gz/Project.toml`
  [4fba245c] + ArrayInterface v6.0.14

julia> using ArrayInterface

julia> u_base = randn(10, 10, 10); u_view = view(u_base, 3, :, :); u_reshaped_view = reshape(u_view, 1, size(u_base, 2) * size(u_base, 3));

julia> ArrayInterface.dense_dims(u_reshaped_view)

julia> ArrayInterface.dense_dims(u_reshaped_view) |> isnothing
true

Thus, this issue is not resolved in general.

[Tokazama]

I guess in that example we could know that no dimensions are dense because u_view doesn't have any dense dimensions, but I'm not sure we can do any better at identifying instances where they are dense.

[Tokazama]

#307 tests all the examples in this issue. I'm assuming there are still times when we don't detect dense dimensions just because we can't know in some situations without knowing the size.