Fix/dataset clean task (#534)

api/tasks/clean_dataset_task.py

 from core.index.index import IndexBuilder
 from extensions.ext_database import db
 from models.dataset import DocumentSegment, Dataset, DatasetKeywordTable, DatasetQuery, DatasetProcessRule, \
-    AppDatasetJoin
+    AppDatasetJoin, Document
 
 
 @shared_task
             index_struct=index_struct
         )
 
-        documents = db.session.query(DocumentSegment).filter(DocumentSegment.dataset_id == dataset_id).all()
+        documents = db.session.query(Document).filter(Document.dataset_id == dataset_id).all()
         segments = db.session.query(DocumentSegment).filter(DocumentSegment.dataset_id == dataset_id).all()
 
         vector_index = IndexBuilder.get_index(dataset, 'high_quality')