Support EPLB in FusedMoE (#8448)

python/sglang/srt/eplb/expert_distribution.py

@@ -47,6 +47,11 @@ class ExpertDistributionRecorder(ABC):
         rank: int,
     ):
         if server_args.expert_distribution_recorder_mode is not None:
+            assert (
+                expert_location_metadata is not None
+            ), "ExpertLocationMetadata is required for expert distribution recording. One possible"
+            "reason is that you are using a model that does not support expert distribution"
+            "recording. Try setting `get_model_config_for_expert_location` in your model."
             return _ExpertDistributionRecorderReal(
                 server_args, expert_location_metadata, rank
             )