[python-package] fix mypy error about missing type hint in dask.py

python-package/lightgbm/dask.py

@@ -930,7 +930,7 @@ def _extract(items: List[Any], i: int) -> Any:
             num_cols = model.n_features_ + 1
 
             nrows_per_chunk = data.chunks[0]
-            out = [[] for _ in range(num_classes)]
+            out: List[dask_Array] = [[] for _ in range(num_classes)]
 
             # need to tell Dask the expected type and shape of individual preds
             pred_meta = data._meta