[XPU] Enable custom routing functions in IPEX for Llama4 (#28004)

Signed-off-by: frost-intel <frost.mitchell@intel.com>

vllm/model_executor/layers/fused_moe/layer.py

@@ -915,7 +915,6 @@ class UnquantizedFusedMoEMethod(FusedMoEMethodBase, CustomOp):
             or logical_replica_count is not None
         ):
             raise NotImplementedError("Expert load balancing is not supported for XPU.")
-        assert custom_routing_function is None
         return layer.ipex_fusion(
             x,
             use_grouped_topk,
@@ -924,6 +923,7 @@ class UnquantizedFusedMoEMethod(FusedMoEMethodBase, CustomOp):
             renormalize,
             topk_group,
             num_expert_group,
+            custom_routing_function=custom_routing_function,
         )
 
     def forward_tpu(