[Bugfix] Fix precision corruption when shared_experts_stream=None (#28942)

Signed-off-by: zhyajie <yajizhan@amd.com>
Co-authored-by: zhyajie <yajizhan@amd.com>

vllm/model_executor/layers/fused_moe/layer.py

@@ -371,8 +371,8 @@ class FusedMoE(CustomOp):
             logger.info_once("Disabling MoE shared_experts cuda stream")
             self.shared_experts_stream = None
         else:
-            # TODO(rob): enable shared expert overlap with non-cuda.
-            # aux_stream() returns None on non-cuda platforms.
+            # TODO(rob): enable shared expert overlap with non-cuda-alike.
+            # aux_stream() returns None on non-cuda-alike platforms.
             self.shared_experts_stream = aux_stream()
             if self.shared_experts_stream is not None:
                 logger.info_once("Enabled separate cuda stream for MoE shared_experts")
@@ -1865,6 +1865,11 @@ class FusedMoE(CustomOp):
                 hidden_states_combined, router_logits = get_ep_group().dispatch(
                     hidden_states, router_logits, self.is_sequence_parallel
                 )
+            # Run shared experts before matrix multiply.
+            # because matrix multiply maybe modify the hidden_states.
+            if has_separate_shared_experts and not use_shared_experts_stream:
+                assert self.shared_experts is not None
+                shared_output = self.shared_experts(hidden_states)
 
             # Matrix multiply.
             final_hidden_states = self.quant_method.apply(
@@ -1908,8 +1913,6 @@ class FusedMoE(CustomOp):
                         # conflict with the main stream
                         shared_output = self.shared_experts(hidden_states_clone)
                     current_stream().wait_stream(self.shared_experts_stream)
-                else:
-                    shared_output = self.shared_experts(hidden_states)
 
                 final_hidden_states = (
                     shared_output,

vllm/utils/torch_utils.py

@@ -426,8 +426,7 @@ def aux_stream() -> torch.cuda.Stream | None:
 
     from vllm.platforms import current_platform
 
-    # TODO: validate this works properly on ROCm platform.
-    if _aux_stream is None and current_platform.is_cuda():
+    if _aux_stream is None and current_platform.is_cuda_alike():
         _aux_stream = torch.cuda.Stream()
 
     return _aux_stream