[BugFix] Add an env to disable moe chunking to work around compile incompatibility (#19642)

Signed-off-by: Ye (Charlotte) Qi <yeq@meta.com>

vllm/model_executor/layers/fused_moe/modular_kernel.py

@@ -225,6 +225,10 @@ class FusedMoEPermuteExpertsUnpermute(ABC):
         else:
             raise ValueError(f"Unsupported FusedMoe activation: {activation}")
 
+    def enable_chunking(self):
+        return envs.VLLM_ENABLE_FUSED_MOE_ACTIVATION_CHUNKING and \
+          self.supports_chunking()
+
     @abstractmethod
     def apply(
         self,
@@ -400,7 +404,7 @@ class FusedMoEModularKernel(torch.nn.Module):
         else:
             _, M, N, K, top_k = _moe_problem_size(a1q, w1, w2, topk_ids)
 
-            if self.fused_experts.supports_chunking():
+            if self.fused_experts.enable_chunking():
                 CHUNK_SIZE = envs.VLLM_FUSED_MOE_CHUNK_SIZE
                 num_chunks = cdiv(M, CHUNK_SIZE)
             else: