Enable loading of fused expert weights in the Transformers modelling backend (#36997)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/model_executor/layers/fused_moe/layer.py

@@ -1342,22 +1342,41 @@ class FusedMoE(CustomOp):
                 weight_name = qual_name.replace(weight_name, param_name)
                 param_name = weight_name.removeprefix(f"{self.layer_name}.")
                 param = getattr(self, param_name)
-                success = self.weight_loader(
-                    param=param,
-                    loaded_weight=loaded_weight,
-                    weight_name=weight_name,
-                    shard_id=shard_id,
-                    expert_id=expert_id,
-                    return_success=True,
-                )
-                if success:
-                    logger.debug(
-                        "Loaded %s for expert %d into %s",
-                        param_name,
-                        expert_id,
-                        self.layer_name,
+                # Fused expert weights can be identified by their 3D tensors
+                if loaded_weight.dim() == 3:
+                    # Repurpose expert_id as shard_idx for deconcatenating w1 and w3
+                    if shard_id in {"w1", "w3"}:
+                        shard_idx = expert_id
+                        experts_shard = loaded_weight.chunk(2, dim=1)[shard_idx]
+                    else:
+                        experts_shard = loaded_weight
+                    start = 0
+                else:
+                    # loaded_weight is a single expert weight, so we add a dummy expert
+                    # dimension to unify the loading logic with the fused case
+                    experts_shard = loaded_weight.unsqueeze(0)
+                    start = expert_id
+
+                # Unified loading logic for fused and non-fused experts
+                loaded_experts = experts_shard.unbind()
+                for expert_id, loaded_expert in enumerate(loaded_experts, start=start):
+                    success = self.weight_loader(
+                        param=param,
+                        loaded_weight=loaded_expert,
+                        weight_name=weight_name,
+                        shard_id=shard_id,
+                        expert_id=expert_id,
+                        return_success=True,
                     )
-                    yield param_name
+                    if success:
+                        logger.debug(
+                            "Loaded expert %d of shard %s into %s for layer %s",
+                            expert_id,
+                            shard_id,
+                            param_name,
+                            self.layer_name,
+                        )
+                        yield param_name
 
     def get_expert_weights(self) -> Iterable[torch.Tensor]:
         def _maybe_make_contiguous(

vllm/model_executor/models/transformers/moe.py

@@ -156,6 +156,17 @@ class MoEMixin(MixtureOfExperts):
         Params for weights, fp8 weight scales, fp8 activation scales
         (param_name, weight_name, expert_id, shard_id)
         """
+        # Models saved with fused experts. These are checkpoints released:
+        # - After Transformers v5
+        # - Before Transformers v5, but re-saved with save_original_format=False
+        # In the fused experts case, we repurpose the expert_id as shard_idx for
+        # deconcatenating w1 and w3 in FusedMoE.load_weights.
+        expert_mapping = [
+            ("experts.w13_weight", "experts.gate_up_proj", 0, "w1"),
+            ("experts.w13_weight", "experts.gate_up_proj", 1, "w3"),
+            ("experts.w2_weight", "experts.down_proj", 0, "w2"),
+        ]
+        # Models saved with ModuleList experts
         ckpt_names = [
             # (ckpt_gate_proj_name, ckpt_down_proj_name, ckpt_up_proj_name)
             ("gate_proj", "down_proj", "up_proj"),  # Most common MoE style
@@ -164,7 +175,6 @@ class MoEMixin(MixtureOfExperts):
         ]
         num_experts = self.model_config.get_num_experts()
         num_redundant_experts = self.parallel_config.eplb_config.num_redundant_experts
-        expert_mapping = []
         for gate_proj, down_proj, up_proj in ckpt_names:
             expert_mapping.extend(
                 FusedMoE.make_expert_params_mapping(