fix kernel output extract (PaddlePaddle#5208)

Author: freeliuzc

fastdeploy/spec_decode/mtp.py

@@ -799,7 +799,7 @@ def _propose(self, step_use_cudagraph: bool = False):
                         self.model_inputs["is_block_step"],
                         self.model_inputs["decode_states"],
                         self.model_inputs["mask_rollback"],
-                    )[0]
+                    )
                     self.model_inputs["attn_mask_offsets"].copy_(attn_mask_offsets, False)
 
                 # Initialize forward meta data