[Refactor] Cuda Graph Runner/Backend Refactor (#23906)

Co-authored-by: BBuf <1182563586@qq.com>
Co-authored-by: Cheng Wan <54331508+ch-wan@users.noreply.github.com>
Co-authored-by: Lianmin Zheng <lianminzheng@gmail.com>

python/sglang/bench_one_batch.py

@@ -77,6 +77,7 @@ from sglang.srt.layers.quantization.fp8_utils import initialize_fp8_gemm_config
 from sglang.srt.managers.schedule_batch import Req, ScheduleBatch
 from sglang.srt.managers.scheduler_components.dp_attn import prepare_mlp_sync_batch_raw
 from sglang.srt.mem_cache.base_prefix_cache import EvictParams
+from sglang.srt.model_executor.cuda_graph_config import Phase
 from sglang.srt.model_executor.forward_batch_info import ForwardBatch
 from sglang.srt.model_executor.model_runner import ModelRunner
 from sglang.srt.sampling.sampling_params import SamplingParams
@@ -944,7 +945,10 @@ def latency_test(
 
 
 def main(server_args, bench_args):
-    server_args.cuda_graph_max_bs = max(bench_args.batch_size)
+    # Post-init write to the legacy cuda_graph_max_bs_decode field would
+    # not propagate to cuda_graph_config; update the decode phase directly.
+    if server_args.cuda_graph_config is not None:
+        server_args.cuda_graph_config[Phase.DECODE].max_bs = max(bench_args.batch_size)
 
     _set_envs_and_config(server_args)