community[patch]: fixed multithreading returning List[List[Documents]] instead of List[Documents] (#20230)

Description: When multithreading is set to True and using the
DirectoryLoader, there was a bug that caused the return type to be a
double nested list. This resulted in other places upstream not being
able to utilize the from_documents method as it was no longer a
`List[Documents]` it was a `List[List[Documents]]`. The change made was
to just loop through the `future.result()` and yield every item.
Issue: #20093
Dependencies: N/A
Twitter handle: N/A

libs/community/langchain_community/document_loaders/directory.py

@@ -174,7 +174,8 @@ class DirectoryLoader(BaseLoader):
                         )
                     )
                 for future in concurrent.futures.as_completed(futures):
-                    yield future.result()
+                    for item in future.result():
+                        yield item
         else:
             for i in items:
                 yield from self._lazy_load_file(i, p, pbar)