auto-detect multi-node based on env vars

[bryevdv]

fixes: #829

This PR add support to modulate the default nodes and ranks_per_node values in the presence of common relevant environment variables. This is most useful when legate is "externally launched" but the --help string is updated with the corresponding information when this occurs:

  --nodes NODES         Number of nodes to use [default auto-detected from OMPI] (default: 4)
  --ranks-per-node RANKS_PER_NODE
                        Number of ranks (processes running copies of the program) to launch per node. [default auto-detected from OMPI] (default: 2)

[bryevdv]

@manopapad note that I do also see:

dev310 ❯ OMPI_COMM_WORLD_SIZE=4 OMPI_COMM_WORLD_LOCAL_SIZE=2 OMPI_COMM_WORLD_RANK=1 legate 
WARNING: Disabling control replication for interactive run
Error: Could not determine node-local rank

but I am not sure that is a real issue (e.g. are other env vars necessary for "real" usage?)

[manopapad]

but I am not sure that is a real issue (e.g. are other env vars necessary for "real" usage?)

That error message is coming from bind.sh I believe. Try also specifying OMPI_COMM_WORLD_RANK and OMPI_COMM_WORLD_LOCAL_RANK.

[bryevdv]

👍 adding OMPI_COMM_WORLD_LOCAL_RANK resolved the bind.sh issue