remove useless code

paddlespeech/s2t/__init__.py

@@ -166,7 +166,6 @@ def broadcast_shape(shp1, shp2):
 def masked_fill(xs: paddle.Tensor,
                 mask: paddle.Tensor,
                 value: Union[float, int]):
-    mask.stop_gradient = True
     mask = mask.astype(xs.dtype)
     return xs * (1.0 - mask) + mask * value