[data] fix loader (#7207)

* fix dataloader

* add test case

* fix type

* fix ci

* fix ci

* fix ci

* disable overwrite cache in ci

tests/data/test_converter.py

@@ -1,3 +1,17 @@
+# Copyright 2025 the LlamaFactory team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
 from llamafactory.data import Role
 from llamafactory.data.converter import get_dataset_converter
 from llamafactory.data.parser import DatasetAttr