bugfix: Fix MLA decode error having v_scale without return_lse (#650)

```
[rank0]:   File "/home/simonmo/.conda/envs/vllm/lib/python3.10/site-packages/flashinfer/decode.py", line 1408, in run
[rank0]:     out[0] *= v_scale
[rank0]: TypeError: 'tuple' object does not support item assignment
```

This is saying `out[0]` does not allow mutation.

python/flashinfer/decode.py

@@ -1404,10 +1404,10 @@ def run(
                 maybe_lse,
                 get_cuda_stream(device),
             )
-            out = (o, maybe_lse) if return_lse else (o,)
+            out = [o, maybe_lse] if return_lse else [o]
         if v_scale is not None:
             out[0] *= v_scale
 
-        return out if return_lse else out[0]
+        return tuple(out) if return_lse else out[0]
 
     run_return_lse = functools.partialmethod(run, return_lse=True)