Support local_rank computation with MPI<3

[jacobhinkle]

Is your feature request related to a problem? Please describe.
Trying to use mpirun less than version 3 with for example lagomorph lddmm atlas results in an error currently since we can't properly find the local rank.

Describe the solution you'd like
Local rank should be determined in a uniform way regardless of MPI version. We should try the method used now, which is what horovod uses, but fall back to a naive hostname-based method if the import fails.

Describe alternatives you've considered
Previously we did not need to compute local rank because we accepted it on the command line as an argument. This is a bit cumbersome however, and switching to computation means the calling convention for lagomorph which uses pytorch.distributed will match horovod.

Additional context
The following stackoverflow response outlines the basic method we need to fall back to:
https://stackoverflow.com/a/31792540
The steps required are:

• On each rank, compute processor name or hostname
• Perform an allgather to grab all of the node names
• sort the unique node names alphabetically
• find integer index of this rank's hostname
• use mpi_comm_split with the integer index found in the last step as the "color"
  This can all be done inside lagomorph.utils.mpi_local_comm