Fall back if flashinfer comm module not found (#20936)

Signed-off-by: Yong Hoon Shin <yhshin@meta.com>

vllm/compilation/collective_fusion.py

@@ -20,10 +20,12 @@ from vllm.utils import direct_register_custom_op
 from .vllm_inductor_pass import VllmInductorPass
 
 if find_spec("flashinfer"):
+    try:
         import flashinfer.comm as flashinfer_comm
-
         flashinfer_comm = (flashinfer_comm if hasattr(
             flashinfer_comm, "trtllm_allreduce_fusion") else None)
+    except ImportError:
+        flashinfer_comm = None
 else:
     flashinfer_comm = None
 from vllm.platforms import current_platform
@@ -411,7 +413,8 @@ class AllReduceFusionPass(VllmInductorPass):
         use_fp32_lamport = self.model_dtype == torch.float32
         if flashinfer_comm is None:
             logger.warning(
-                "Flashinfer is not installed, skipping allreduce fusion pass")
+                "Flashinfer is not installed or comm module not found, "
+                "skipping allreduce fusion pass")
             return
         # Check if the world size is supported
         if self.tp_size not in _FI_MAX_SIZES: