Update multi-label document and conversion script (#2358)

Update docs, label convert script

Author: sungmanc

docs/source/guide/explanation/algorithms/classification/multi_label_classification.rst

@@ -24,7 +24,7 @@ Dataset Format
 **************
 
 As it is a common practice to use object detection datasets in the academic area, we support the most popular object detection format: `COCO <https://cocodataset.org/#format-data>`_.
-Specifically, this format should be converted in our `internal representation <https://github.com/openvinotoolkit/training_extensions/tree/develop/data/datumaro_multilabel>`_ first. We provided a `script <https://github.com/openvinotoolkit/training_extensions/blob/develop/src/otx/algorithms/classification/utils/convert_coco_to_multilabel.py>` to help with conversion.
+Specifically, this format should be converted in our `internal representation <https://github.com/openvinotoolkit/training_extensions/tree/develop/tests/assets/datumaro_multilabel>`_ first. We provided a `script <https://github.com/openvinotoolkit/training_extensions/blob/develop/src/otx/algorithms/classification/utils/convert_coco_to_multilabel.py>` to help with conversion.
 To convert the COCO data format to our internal one, run this script in similar way:
 
 .. code-block::
@@ -35,6 +35,9 @@ To convert the COCO data format to our internal one, run this script in similar
 
     Please, refer to our :doc:`dedicated tutorial <../../../tutorials/base/how_to_train/classification>` for more information how to train, validate and optimize classification models.
 
+.. note::
+    For now, "___" is a symbol to distinguish the multi-label format. So, it must be included at the front of the label name. 
+
 ******
 Models
 ******

src/otx/algorithms/classification/utils/convert_coco_to_multilabel.py

@@ -61,7 +61,7 @@ def coco_to_datumaro_multilabel(ann_file_path: str, data_root_dir: str, output:
     overall_classes: List = coco_dataset.get_classes()
     for class_name in overall_classes:
         multilabel_ann_format["categories"]["label"]["label_groups"].append(
-            {"name": str(class_name), "group_type": "exclusive", "labels": [str(class_name)]}
+            {"name": f"___{str(class_name)}", "group_type": "exclusive", "labels": [str(class_name)]}
         )
 
         multilabel_ann_format["categories"]["label"]["labels"].append(