add callback to entity extraction (#1843)

### What problem does this PR solve?

### Type of change

- [x] Refactoring
- [ ]

graphrag/index.py

     for i in range(len(chunks)):
         tkn_cnt = num_tokens_from_string(chunks[i])
         if cnt+tkn_cnt >= left_token_count and texts:
-            threads.append(exe.submit(ext, texts, {"entity_types": entity_types}, callback))
+            for b in range(0, len(texts), 16):
+                threads.append(exe.submit(ext, ["\n".join(texts[b:b+16])], {"entity_types": entity_types}, callback))
             texts = []
             cnt = 0
         texts.append(chunks[i])
         cnt += tkn_cnt
     if texts:
-        threads.append(exe.submit(ext, texts))
+        for b in range(0, len(texts), 16):
+            threads.append(exe.submit(ext, ["\n".join(texts[b:b+16])], {"entity_types": entity_types}, callback))
 
     callback(0.5, "Extracting entities.")
     graphs = []