Support trtllm_allreduce_fusion in flashinfer for cuda<12.8 (#9339)

Co-authored-by: Zhang Kaihong <zhangkaihong.zkh@alibaba-inc.com>

python/sglang/srt/layers/layernorm.py

@@ -27,6 +27,7 @@ from sglang.srt.utils import (
     is_cuda,
     is_hip,
     is_npu,
+    supports_custom_op,
 )
 
 _is_cuda = is_cuda()
@@ -202,8 +203,14 @@ class RMSNorm(CustomOp):
                 flashinfer_allreduce_residual_rmsnorm,
             )
 
+            fused_op = (
+                torch.ops.sglang.flashinfer_allreduce_residual_rmsnorm
+                if supports_custom_op()
+                else flashinfer_allreduce_residual_rmsnorm
+            )
+
             if get_tensor_model_parallel_world_size() > 1:
-                fused_result = flashinfer_allreduce_residual_rmsnorm(
+                fused_result = fused_op(
                     input_tensor=x,
                     residual=residual,
                     weight=self.weight,