Feat: text file support position retaining. (#6231)

### What problem does this PR solve?

#5832

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

rag/nlp/__init__.py

 def tokenize_chunks(chunks, doc, eng, pdf_parser=None):
     res = []
     # wrap up as es documents
-    for ck in chunks:
+    for ii, ck in enumerate(chunks):
         if len(ck.strip()) == 0:
             continue
         logging.debug("-- {}".format(ck))
                 ck = pdf_parser.remove_tag(ck)
             except NotImplementedError:
                 pass
+        else:
+            add_positions(d, [[ii]*5])
         tokenize(d, ck, eng)
         res.append(d)
     return res