[Bugfix]Reject invalid FlashInfer MNNVL workspaces (#49043)

Signed-off-by: lengrongfu <lenronfu@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

vllm/distributed/device_communicators/flashinfer_all_reduce.py

@@ -67,6 +67,12 @@ def _create_workspace(
             comm_backend=comm_backend,
             group=group,
         )
+        if backend == "mnnvl" and not getattr(workspace, "mc_ptr", 0):
+            workspace.destroy()
+            logger.warning_once(
+                "FlashInfer MNNVL multicast is unavailable on the current topology."
+            )
+            return None
     except Exception as e:
         if "multicast" in str(e).lower():
             logger.warning_once(