[Bug] Fix `returned_lse` not Defined issue (#25106)

Signed-off-by: yewentao256 <zhyanwentao@126.com>
Co-authored-by: Tyler Michael Smith <tyler@neuralmagic.com>

vllm/v1/attention/backends/mla/cutlass_mla.py

@@ -206,12 +206,11 @@ class CutlassMLAImpl(MLACommonImpl[MLACommonMetadata]):
         )
 
         if H < MAX_HEADS:
-            # Extract the subsets of the outputs
-            returned_lse = lse[:, :H].contiguous(
-            ) if self.need_to_return_lse_for_decode else lse
             out = out[:, :H]
+            if self.need_to_return_lse_for_decode:
+                lse = lse[:, :H].contiguous()
 
-        return out, returned_lse
+        return out, lse
 
     def _forward_decode(
         self,