[Fix] Fall back to the process-group broadcast for DSA topk when PyNCCL is absent (#36963)

python/sglang/srt/layers/attention/dsa/dsa_indexer.py

@@ -154,11 +154,14 @@ def _broadcast_indexer_topk_from_rank0_impl(topk_indices: torch.Tensor) -> None:
     if group.world_size == 1:
         return
 
-    if topk_indices.device.type == "cuda" and torch.cuda.is_current_stream_capturing():
-        if group.pynccl_comm is None:
-            raise RuntimeError(
-                "SGLANG_DSA_TOPK_BROADCAST requires PyNCCL during CUDA graph capture."
-            )
+    # PyNCCL is the faster path under capture, but it is not a precondition:
+    # a split attn-TP group is built without one (parallel_state.py), and the
+    # process-group broadcast captures and replays correctly.
+    if (
+        topk_indices.device.type == "cuda"
+        and torch.cuda.is_current_stream_capturing()
+        and group.pynccl_comm is not None
+    ):
         with group.pynccl_comm.change_state(enable=True):
             group.pynccl_comm.broadcast(topk_indices, src=0)
     else: