Add the benchmark flow for ATOM vLLM plugin

[wuhuikx]

1. This PR adds a reproducible benchmark flow for ATOM vLLM Plugin. The default environment is rocm/atom-dev:vllm-latest with the benchmarking recipe aligned with InferenceMax and ATOM itself.

2. It also provides an option to compare ATOM vLLM plugin against upstream vLLM under a fixed A/B setup. It standardizes benchmark execution and reports key serving metrics (throughput + latency) for direct runtime comparison.

Here is an example to compare performance between ATOM vLLM Plugin and Upstream vllm (with vllm/vllm-openai-rocm:nightly) for Kimi-K2-Thinking-MXFP4 (ISL/OSL=1K/1K, concurrency=8, TP=4).

Key Results (1K/1K, C8, TP4)

1k/1k c8 主结果（TP4）
ATOM plugin

Output TPUT: 578.85 tok/s
Total TPUT: 1152.17 tok/s
mean TTFT: 135.47 ms
mean TPOT: 12.97 ms
mean E2EL: 13898.61 ms
Upstream nightly

Output TPUT: 459.33 tok/s
Total TPUT: 914.28 tok/s
mean TTFT: 225.03 ms
mean TPOT: 16.38 ms
mean E2EL: 17570.20 ms
对比结论（ATOM 相对 upstream）
吞吐：
Output TPUT +26.02%
Total TPUT +26.02%
时延（越低越好）：
TTFT -39.80%（更低）
TPOT -20.82%
E2EL -20.90%