[Bugfix] Gracefully disable AllReduceFusionPass on GPUs without multicast support (#35085)

Signed-off-by: haosdent <haosdent@gmail.com>

vllm/compilation/passes/fusion/allreduce_rms_fusion.py

@@ -729,6 +729,7 @@ class AllReduceFusionPass(VllmPatternMatcherPass):
             scope="global",
         )
 
+        try:
             self.workspace = flashinfer_comm.create_allreduce_fusion_workspace(
                 backend="trtllm",
                 world_size=self.tp_size,
@@ -737,6 +738,17 @@ class AllReduceFusionPass(VllmPatternMatcherPass):
                 hidden_dim=self.hidden_dim,
                 dtype=self.model_dtype,
             )
+        except RuntimeError as e:
+            if "multicast" not in str(e).lower():
+                raise
+            logger.warning_once(
+                "AllReduce fusion pass is disabled: flashinfer workspace "
+                "creation failed: %s. This is expected on GPUs without "
+                "NVSwitch (e.g., NVLink bridge-only or PCIe topologies). "
+                "Falling back to non-fused allreduce.",
+                str(e),
+            )
+            return
 
         global _FI_WORKSPACE
         _FI_WORKSPACE = self.workspace