Fix: change the chatgpt plugin retriever metadata format (#5920)

the current implement put the doc itself as the metadata, but the
document chatgpt plugin retriever returned already has a `metadata`
field, it's better to use that instead.

the original code will throw the following exception when using
`RetrievalQAWithSourcesChain`, becuse it can not find the field
`metadata`:

```python
Exception has occurred: ValueError       (note: full exception trace is shown but execution is paused at: _run_module_as_main)
Document prompt requires documents to have metadata variables: ['source']. Received document with missing metadata: ['source'].
  File "/home/wangjie/anaconda3/envs/chatglm/lib/python3.10/site-packages/langchain/chains/combine_documents/base.py", line 27, in format_document
    raise ValueError(
  File "/home/wangjie/anaconda3/envs/chatglm/lib/python3.10/site-packages/langchain/chains/combine_documents/stuff.py", line 65, in <listcomp>
    doc_strings = [format_document(doc, self.document_prompt) for doc in docs]
  File "/home/wangjie/anaconda3/envs/chatglm/lib/python3.10/site-packages/langchain/chains/combine_documents/stuff.py", line 65, in _get_inputs
    doc_strings = [format_document(doc, self.document_prompt) for doc in docs]
  File "/home/wangjie/anaconda3/envs/chatglm/lib/python3.10/site-packages/langchain/chains/combine_documents/stuff.py", line 85, in combine_docs
    inputs = self._get_inputs(docs, **kwargs)
  File "/home/wangjie/anaconda3/envs/chatglm/lib/python3.10/site-packages/langchain/chains/combine_documents/base.py", line 84, in _call
    output, extra_return_dict = self.combine_docs(
  File "/home/wangjie/anaconda3/envs/chatglm/lib/python3.10/site-packages/langchain/chains/base.py", line 140, in __call__
    raise e
```

Additionally, the `metadata` filed in the `chatgpt plugin retriever`
have these fileds by default:
```json
{
    "source":  "file",   //email, file or chat
    "source_id": "filename.docx", // the filename
    "url": "", 
    ...
}
```
so, we should set `source_id` to `source` in the langchain metadata.

```python
metadata = d.pop("metadata", d)
if(metadata.get("source_id")):
    metadata["source"] = metadata.pop("source_id")
```

#### Who can review?
@dev2049

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---------

Co-authored-by: wangjie <wangjie@htffund.com>

langchain/retrievers/chatgpt_plugin_retriever.py

@@ -28,7 +28,10 @@ class ChatGPTPluginRetriever(BaseRetriever, BaseModel):
         docs = []
         for d in results:
             content = d.pop("text")
-            docs.append(Document(page_content=content, metadata=d))
+            metadata = d.pop("metadata", d)
+            if metadata.get("source_id"):
+                metadata["source"] = metadata.pop("source_id")
+            docs.append(Document(page_content=content, metadata=metadata))
         return docs
 
     async def aget_relevant_documents(self, query: str) -> List[Document]:
@@ -48,7 +51,10 @@ class ChatGPTPluginRetriever(BaseRetriever, BaseModel):
         docs = []
         for d in results:
             content = d.pop("text")
-            docs.append(Document(page_content=content, metadata=d))
+            metadata = d.pop("metadata", d)
+            if metadata.get("source_id"):
+                metadata["source"] = metadata.pop("source_id")
+            docs.append(Document(page_content=content, metadata=metadata))
         return docs
 
     def _create_request(self, query: str) -> tuple[str, dict, dict]: