bugfix: Fix the default value of data_type in batch decode plan fun…

…ction (#544)

As mentioned in #543 , the behavior of batch decode plan function is
problematic if we use the combo of `q_data_type` and `kv_data_type`.

cc @cyang49

python/flashinfer/decode.py

@@ -499,9 +499,9 @@ def plan(
         pos_encoding_mode: str = "NONE",
         window_left: int = -1,
         logits_soft_cap: Optional[float] = None,
-        data_type: Optional[Union[str, torch.dtype]] = "float16",
         q_data_type: Optional[Union[str, torch.dtype]] = "float16",
         kv_data_type: Optional[Union[str, torch.dtype]] = None,
+        data_type: Optional[Union[str, torch.dtype]] = None,
         sm_scale: Optional[float] = None,
         rope_scale: Optional[float] = None,
         rope_theta: Optional[float] = None,