[Bugfix] Add method to swap quant_method on FusedMoE to fix LoRA issues (#34453)

Signed-off-by: Bill Nell <bnell@redhat.com>

vllm/lora/layers/fused_moe.py

@@ -338,8 +338,9 @@ class FusedMoEWithLoRA(BaseLayerWithLoRA):
         fused_experts.moe_sum = moe_sum_decorator(
             self.base_layer, fused_experts.moe_sum
         )
-        self.base_layer.quant_method = FusedMoEModularMethod(
-            self.base_layer.quant_method, m_fused_moe_fn
+        # TODO(bnell): find a less intrusive way to handle this.
+        self.base_layer._replace_quant_method(
+            FusedMoEModularMethod(self.base_layer.quant_method, m_fused_moe_fn)
         )
 
     def _create_lora_a_weights(

vllm/model_executor/layers/fused_moe/layer.py

@@ -655,6 +655,16 @@ class FusedMoE(CustomOp):
             enable_dbo=self.vllm_config.parallel_config.enable_dbo,
         )
 
+    # TODO(bnell): This method is provided as a hook so vllm/lora/layers/fused_moe.py
+    # can safely swap out the quant_method. We should figure out a less
+    # intrusive way to do this.
+    def _replace_quant_method(self, mk: FusedMoEMethodBase):
+        self.quant_method = mk
+        # We need to force reconstruction of runner because we're swapping out
+        # the quant_method with a FusedMoEModularMethod. This logic can go
+        # away once the FusedMoEModularMethod is eliminated.
+        self.runner = self._init_runner()
+
     # Note: maybe_init_modular_kernel should only be called by
     # prepare_communication_buffer_for_model.
     # This is called after all weight loading and post-processing, so it
@@ -676,17 +686,15 @@ class FusedMoE(CustomOp):
             logger.debug(
                 "%s for %s(%s)", prepare_finalize.__class__.__name__, self, id(self)
             )
-            self.quant_method = FusedMoEModularMethod.make(
-                self,
-                self.quant_method,
-                prepare_finalize,
-                self.shared_experts,
-                inplace=not self.moe_config.disable_inplace,
+            self._replace_quant_method(
+                FusedMoEModularMethod.make(
+                    self,
+                    self.quant_method,
+                    prepare_finalize,
+                    self.shared_experts,
+                    inplace=not self.moe_config.disable_inplace,
+                )
             )
-            # We need to force reconstruction of runner because we're swapping out
-            # the quant_method with a FusedMoEModularMethod. This logic can go
-            # away once the FusedMoEModularMethod is eliminated.
-            self.runner = self._init_runner()
 
     @property
     def shared_experts(self) -> torch.nn.Module | None: