layout() function broken for Views with padding -- path forward

[crtrott]

We have an issue because the LayoutLeft and LayoutRight class itself (not the mapping) has no place to store stride. As a consequence the following code is wrong:

Kokkos::View<int**,Kokkos::LayoutLeft> a("A",N,M);
Kokkos::View<int**,Kokkos::LayoutLeft> a_sub =
  Kokkos::subview(a, Kokkos::pair{3,5},Kokkos::ALL());

Kokkos::View<int**,Kokkos::LayoutLeft> b_sub(a.sub.data(), a_sub.layout());
//b_sub.stride(1) != a_sub.stride(1);

Suggested path forward:

• restrict layout() to Kokkos::LayoutLeft/Right/Stride and deprecate
  • error out when calling layout() where stride!=extent
• introduce View::mapping() -> return mdspan mappings
• introduce View(data_handle, mdspan_mapping_type map)

[masterleinad]

It's actually not stride(1) but stride(2) that fails: https://godbolt.org/z/14q5o136r

[crtrott]

Because the code should have used a_sub.layout() corrected.

[crtrott]

Also accessing stride(2) on a rank-2 view is kinda UB lol