[GPU] Multinode support for multihost_hlo_runner

[trevor-m]

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This PR adds multinode support to the multihost_hlo_runner. To use, there is a new command line argument address for the address of the coordinator/root node.

Example usage with SLURM:

bazel run //xla/tools/multihost_hlo_runner:hlo_runner_main -- \
  --task_id=${SLURM_PROCID} \
  --num_nodes=${SLURM_NTASKS} \
  --address="${SLURM_LAUNCH_NODE_IPADDR}:12345" \
  ...

[trevor-m]

@PatriosTheGreat

[cheshire]

Looks good to me, @ezhulenev @hawkinsp any thoughts?