Skip redundant moe_sum_reduce for single-expert routing on XPU (#22660)

Co-authored-by: Ma Mingfei <mingfei.ma@intel.com>

python/sglang/srt/layers/moe/moe_runner/triton_utils/fused_moe.py

@@ -790,11 +790,14 @@ def _fused_moe_kernel_sequence(
                     routed_scaling_factor,
                 )
     elif _is_xpu:
-        moe_sum_reduce(
-            intermediate_cache3.view(*intermediate_cache3.shape),
-            out_hidden_states,
-            routed_scaling_factor,
-        )
+        if topk == 1 and routed_scaling_factor == 1.0 and not _use_intermediate:
+            pass  # we wrote directly into out_hidden_states
+        else:
+            moe_sum_reduce(
+                intermediate_cache3.view(*intermediate_cache3.shape),
+                out_hidden_states,
+                routed_scaling_factor,
+            )
     else:
         if _has_vllm_ops:
             vllm_ops.moe_sum(