[Bugfix] Fix GPT-OSS AR+NORM fusion (#28841)

Signed-off-by: elvischenv <219235043+elvischenv@users.noreply.github.com>

.buildkite/test-pipeline.yaml

@@ -971,6 +971,7 @@ steps:
   - vllm/model_executor/layers/layernorm.py
   - vllm/model_executor/layers/activation.py
   - vllm/model_executor/layers/quantization/input_quant_fp8.py
+  - vllm/model_executor/layers/fused_moe/layer.py
   - tests/compile/test_fusion_attn.py
   - tests/compile/test_silu_mul_quant_fusion.py
   - tests/compile/distributed/test_fusion_all_reduce.py

tests/compile/distributed/test_fusions_e2e.py

@@ -111,6 +111,17 @@ if current_platform.is_cuda():
                 async_tp=96,  # MLP is MoE, half the fusions of dense
             ),
         ),
+        ModelBackendTestCase(
+            model_name="openai/gpt-oss-20b",
+            model_kwargs=dict(max_model_len=1024, kv_cache_dtype="fp8"),
+            backend=AttentionBackendEnum.FLASHINFER,
+            matches=Matches(
+                attention_fusion=0,
+                allreduce_fusion=49,
+                sequence_parallel=49,
+                async_tp=48,
+            ),
+        ),
     ]
 
 elif current_platform.is_rocm():

vllm/distributed/device_communicators/symm_mem.py

@@ -131,7 +131,7 @@ class SymmMemCommunicator:
             return None
         if out is None:
             out = torch.empty_like(inp)
-        self.buffer[: inp.numel()].copy_(inp.view(-1))
+        self.buffer[: inp.numel()].copy_(inp.reshape(-1))
 
         # Determine which algorithm to use
         use_multimem = False

vllm/model_executor/layers/fused_moe/layer.py

@@ -1690,6 +1690,10 @@ class FusedMoE(CustomOp):
             )
 
         def reduce_output(states: torch.Tensor) -> torch.Tensor:
+            # Slice before all_reduce to enable possible fusion
+            if self.hidden_size != og_hidden_states:
+                states = states[..., :og_hidden_states]
+
             if (
                 not self.is_sequence_parallel
                 and not self.use_dp_chunking
@@ -1712,11 +1716,12 @@ class FusedMoE(CustomOp):
             if self.zero_expert_num is not None and self.zero_expert_num > 0:
                 assert isinstance(fused_output, tuple)
                 fused_output, zero_expert_result = fused_output
-                return (reduce_output(fused_output) + zero_expert_result)[
-                    ..., :og_hidden_states
-                ]
+                return (
+                    reduce_output(fused_output)
+                    + zero_expert_result[..., :og_hidden_states]
+                )
             else:
-                return reduce_output(fused_output)[..., :og_hidden_states]
+                return reduce_output(fused_output)
         else:
             if current_platform.is_tpu():
                 # TODO: Once the OOM issue for the TPU backend is resolved, we
@@ -1729,8 +1734,8 @@ class FusedMoE(CustomOp):
                     hidden_states, router_logits, self.layer_name
                 )
             return (
-                reduce_output(shared_output)[..., :og_hidden_states],
-                reduce_output(fused_output)[..., :og_hidden_states],
+                reduce_output(shared_output),
+                reduce_output(fused_output),
             )
 
     def forward_cuda(