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COLL framework needs work to support MPI Bigcount #12336

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@hppritcha

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@hppritcha

In the course of work done in #12226 it was discovered that, unlike the PML API, the COLL API is not ready for big count.
One option would be to extend the existing table of coll methods to have entry points for large count functions. This would have the plus of only having to implement initially support for big count in the basic and maybe tuned components. It would have the downside of roughly doubling the size of mca_coll_base_comm_coll_t struct. Changing the definitions for all the existing methods to be generalized to support big count would have the down side of needing to go into every existing component and making sure their collective methods can handle MPI_Count and MPI_Aint - and if they can't be modified to support big count, disqualify their implementation of that particular collective operation.

Related to issue #9194 and PR #12226.

We do not plan to include this work in PR #12226 as its already complex enough and is really targeted at the infrastructure for generating the _c MPI API c entry points for big count and the way too long in implementation correct TS 29113 entry points for Fortran F08 (along with support for Big count on the fortran side too).

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