[DOC]: CPP/OMP/TBB backend execution policies missing from new documentation

[pauleonix]

While it seems to be still possible to use e.g. thrust::tbb::par, I can't find any mention of these execution policies in the new or old documentation, even though the files like e.g. thrust/system/tbb/execution_policy.h do encompass doxygen comments. The old github wiki pages like https://github.com/thrust/thrust/wiki/Host-Backends have vanished as well.
Instead users will only find how to change host and device backends via CMake or compiler flags.

Is the use of these execution policies deprecated or discouraged? Are we waiting for someone to write better documentation? Is it just a bug in the documantation that the execution_policy.h pages don't appear? I couldn't find another issue or PR discussing this.

[pca006132]

Also, if these execution policies are not deprecated/discouraged to use, is it legitimate to mix them? I mixed thrust::cpp::par (which should be sequential I think?), thrust::tbb::par/thrust::omp::par and thrust::cuda::par in another project to implement dynamic backend (#130 but it seems dead). This works on my machine on on our tests, but want to be sure if this is expected to work or are we discouraged from doing so.

[alliepiper]

Using the explicit policies for cpp/tbb/omp is okay.

The old wiki was taken down a while ago since it was mostly out of date, but a copy of it can be found here: https://github.com/brycelelbach/thrust_wiki

The execution_policy docs missing from the new docs is a bug, we should fix this. Let's use this issue to track it.

[pauleonix]

Thanks for pointing out the new link to the old wiki. Especially the content of https://github.com/brycelelbach/thrust_wiki/blob/main/Direct-System-Access.md should be integrated into the official docs.

[pauleonix]

@jrhemstad This one could also be included in NVIDIA/thrust#1875