arxiv downloader return Dataset instead of DJDataset

data_juicer/download/arxiv.py

@@ -5,10 +5,10 @@
 import tarfile
 import tempfile
 
+from datasets import Dataset
 from downloader import (DocumentDownloader, DocumentExtractor,
                         DocumentIterator, download_and_extract, get_arxiv_urls)
 
-from data_juicer.core.data import DJDataset
 from data_juicer.utils.file_utils import (expand_outdir_and_mkdir,
                                           get_all_files_paths_under)
 
@@ -355,7 +355,7 @@ def download_arxiv(
     keep_raw_download=False,
     force_download=False,
     url_limit=None,
-) -> DJDataset:
+) -> Dataset:
     """
     Downloads Arxiv tar files and extracts them