[XLA:GPU] Support mocking multi-gpu execution from a single GPU

[copybara-service[bot]]

[XLA:GPU] Support mocking multi-gpu execution from a single GPU

Allows running multi-GPU HLOs on a machine with a single GPU, when --enable_mock_nccl is used.

Example usage:

./tools/multihost_hlo_runner/hlo_runner_main  --device_type=gpu --use_spmd_partitioning --num_partitions=16 --num_replicas=1 --enable_mock_nccl multi_gpu.hlo