Turn on custom allreduce by default in O3 (#2706)

Author: vinx13

python/mlc_llm/interface/compiler_flags.py

@@ -205,5 +205,6 @@ def from_str(source: str) -> "ModelConfigOverride":
         faster_transformer=True,
         cudagraph=True,
         cutlass=True,
+        ipc_allreduce_strategy=IPCAllReduceStrategyType.AUTO,
     ),
 }