Fix: change the chatgpt plugin retriever metadata format

[JaysonAlbert]

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:

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:

{
    "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.

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

Who can review?

@dev2049

[hwchase17]

lgtm - thanks