Add function to convert mdspan to a const view

[lowener]

make_const_mdspan is a helper function to convert mdspan<T> into mdspan<const T>.
I added examples of it's usage
@mhoemmen @Nyrio

[Nyrio]

Thanks for this!

[mhoemmen]

Thanks for writing this!

This facility serves an analogous purpose as std::view::as_const.

https://en.cppreference.com/w/cpp/ranges/as_const_view

The use case example here

https://en.cppreference.com/w/cpp/ranges/constant_range

looks very much like the use cases you would like to support. The idea is that you would like to use mdspan<const T, ...> to express that an mdspan parameter is a view-of-const, but you would like the function nevertheless to be callable by users with mdspan<T, ...>.

[codecov-commenter]

Codecov Report

Base: 87.99% // Head: 87.99% // No change to project coverage 👍

Coverage data is based on head (3e3facb) compared to base (92820d5).
Patch has no changes to coverable lines.

Additional details and impacted files

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[mhoemmen]

Thanks for considering this design change! I find that this makes the code clean and easy to extend.

Optionally, if you want users to be able to add support for their custom accessors, you might want to consider a customization point object or tag_invoke design for accessor_of_const.

[mhoemmen]

This is the correct way to do it, as long as host_device_accessor has an element type converting constructor like default_accessor does (see the first constructor here). A straightforward unit test would fail to compile without that constructor.

[cjnolet]

This is a great point, @mhoemmen. It's nice to see the new function being used in the k-means tests but we should probably have a dedicated testcase for this (ideally in the mdspan cpp test file).

[cjnolet]

/merge