[ci] [python-package] fix mypy errors about Dataset._set_init_score_by_predictor() (#5850)

python-package/lightgbm/basic.py

@@ -1744,9 +1744,11 @@ class Dataset:
             data_has_header = any(self.params.get(alias, False) for alias in _ConfigAliases.get("header"))
         num_data = self.num_data()
         if predictor is not None:
-            init_score = predictor.predict(data,
-                                           raw_score=True,
-                                           data_has_header=data_has_header)
+            init_score: Union[np.ndarray, scipy.sparse.spmatrix] = predictor.predict(
+                data=data,
+                raw_score=True,
+                data_has_header=data_has_header
+            )
             init_score = init_score.ravel()
             if used_indices is not None:
                 assert not self._need_slice
@@ -1765,7 +1767,7 @@ class Dataset:
                         new_init_score[j * num_data + i] = init_score[i * predictor.num_class + j]
                 init_score = new_init_score
         elif self.init_score is not None:
-            init_score = np.zeros(self.init_score.shape, dtype=np.float64)
+            init_score = np.full_like(self.init_score, fill_value=0.0, dtype=np.float64)
         else:
             return self
         self.set_init_score(init_score)