[MPI_Intercomm_create] Peer_comm intercommunicator is now allowed

[EmmanuelBRELLE]

MPI 5.0 specification (section 7.6) allows MPI_Intercomm_create peer_comm to be either an intracommunicator or any kind of communicator.
In the current implementation this communicator is only used by local root in PML send/recv communications.
Unless I missed something, I should be OK to release the intracommunicator constrain.

[github-actions]

Hello! The Git Commit Checker CI bot found a few problems with this PR:

6428260: [MPI_Intercomm_create] Peer_comm intercommunicator...

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Please fix these problems and, if necessary, force-push new commits back up to the PR branch. Thanks!

[bosilca]

You are correct, the standard places no restrictions on the peer communicato for as long as the two leaders can communicate through it, which means being in different groups.