[Core] Rank-to-device mapping env var

[BKitor]

Introduce VLLM_LOCAL_RANK_DEV_MAP to provide a way to assign devices to ranks.
Currently, each device is assigned based on the worker's local_rank. This is fine for systems were all device-to-device routs are identical, but this isn't the true for all systems.

To provide flexibility, the device-to-rank mapping is specified as a JSON object.
This PR implements the following format for mapping TP/PP jobs.

'{ "tp-0" : "[d_tp0-0, d_tp0-1, ..., d_tp0-(npp-1)]",
   "tp-1" : "[d_tp1-0, d_tp1-1, ..., d_tp1-(npp-1)]",
      ...
   "tp-(ntp-1)" : "[d_tpn-0, d_tpn-1, ..., d_tpn-(npp-1)]" }'
Where npp == pipeline-parallel world size
      ntp == tensor-parallel world size
      d_tpx-y is the device ordinal to map to tp-rank x / pp-rank y

Ideally, this format should be extendable to other multi-GPU workloads like data-parallelism (#9206), speculative decoding or disaggregated prefill.

PR also adds benchmark_communication.py which benchmarks communication calls used by parallel_state.py's GroupCoordinator.

[github-actions]

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