add PRTE_MCA_rmaps_base_mapping_policy

[smoors]

fixes #297

draft for now: not yet tested

[satishskamath]

open-mpi/ompi#13586 (comment)

This option of specifying PEs per slot was removed for the default mapping policy within OMPIv5. This is now being added back and will be merged in further OMPI updates but not the existing versions. I have asked for an equivalent environment variable for the command line option --map-by because we can definitely do it there.

[casparvl]

So... one option I see is to query the launcher and inject the right --map-by arguments (if the launcher is mpirun). The downside is: it needs to be OpenMPI's mpirun, and I'm not sure how EESSI it is to determine that (at least I dont think Intel's mpirun would know this argument?)

[smoors]

apparently intel mpirun happily ignores --map-by (but not --report-bindings), so it's doable

$ mpirun --map-by slot:PE=2 -genv I_MPI_DEBUG=4 hostname                                                                                                                                                             
Options -binding/-bind-to/-map-by/-membind are not supported and will be ignored.
        Please refer I_MPI_PIN environment variables family.
node800.hydra.os
node800.hydra.os
node800.hydra.os
node800.hydra.os

EDIT: this only works for 2025a, it fails in 2024a