opea/llm-faqgen-tgi fails with false streaming setting

[lianhao]

When run the comps/llms/faq-generation/tgi/langchain docker container, if we pass the "streaming": false parameter in the curl request:

curl http://localhost:9000/v1/faqgen -X POST -d '{"query":"Text Embeddings Inference (TEI) is a toolkit for deploying and serving open source text embeddings and sequence classification models. TEI enables high-performance extraction for the most popular models, including FlagEmbedding, Ember, GTE and E5.","streaming":false}' -H 'Content-Type: application/json',

we'll find the curl returns "Internal Server Error" and the following logs in the opea/llm-faqgen-tgi container:

INFO:     127.0.0.1:48474 - "POST /v1/faqgen HTTP/1.1" 500 Internal Server Error                                                                                                    [31/1913]
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/home/user/.local/lib/python3.11/site-packages/uvicorn/protocols/http/h11_impl.py", line 406, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 70, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/lib/python3.11/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "/home/user/.local/lib/python3.11/site-packages/starlette/applications.py", line 113, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/home/user/.local/lib/python3.11/site-packages/starlette/middleware/errors.py", line 187, in __call__
    raise exc
  File "/home/user/.local/lib/python3.11/site-packages/starlette/middleware/errors.py", line 165, in __call__
    await self.app(scope, receive, _send)
  File "/home/user/.local/lib/python3.11/site-packages/prometheus_fastapi_instrumentator/middleware.py", line 174, in __call__
    raise exc
  File "/home/user/.local/lib/python3.11/site-packages/prometheus_fastapi_instrumentator/middleware.py", line 172, in __call__
    await self.app(scope, receive, send_wrapper)
  File "/home/user/.local/lib/python3.11/site-packages/starlette/middleware/cors.py", line 85, in __call__
    await self.app(scope, receive, send)
  File "/home/user/.local/lib/python3.11/site-packages/starlette/middleware/exceptions.py", line 62, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "/home/user/.local/lib/python3.11/site-packages/starlette/_exception_handler.py", line 62, in wrapped_app
    raise exc
  File "/home/user/.local/lib/python3.11/site-packages/starlette/_exception_handler.py", line 51, in wrapped_app
    await app(scope, receive, sender)
  File "/home/user/.local/lib/python3.11/site-packages/starlette/routing.py", line 715, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/home/user/.local/lib/python3.11/site-packages/starlette/routing.py", line 735, in app
    await route.handle(scope, receive, send)
  File "/home/user/.local/lib/python3.11/site-packages/starlette/routing.py", line 288, in handle
    await self.app(scope, receive, send)
  File "/home/user/.local/lib/python3.11/site-packages/starlette/routing.py", line 76, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "/home/user/.local/lib/python3.11/site-packages/starlette/_exception_handler.py", line 62, in wrapped_app
    raise exc
  File "/home/user/.local/lib/python3.11/site-packages/starlette/_exception_handler.py", line 51, in wrapped_app
    await app(scope, receive, sender)
  File "/home/user/.local/lib/python3.11/site-packages/starlette/routing.py", line 73, in app
    response = await f(request)
               ^^^^^^^^^^^^^^^^
  File "/home/user/.local/lib/python3.11/site-packages/fastapi/routing.py", line 301, in app
    raw_response = await run_endpoint_function(
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/lib/python3.11/site-packages/fastapi/routing.py", line 214, in run_endpoint_function
    return await run_in_threadpool(dependant.call, **values)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/lib/python3.11/site-packages/starlette/concurrency.py", line 39, in run_in_threadpool
    return await anyio.to_thread.run_sync(func, *args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/anyio/to_thread.py", line 56, in run_sync
    return await get_async_backend().run_sync_in_worker_thread(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 2134, in run_sync_in_worker_thread
    return await future
           ^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/anyio/_backends/_asyncio.py", line 851, in run
    result = context.run(func, *args)
             ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/comps/llms/faq-generation/tgi/llm.py", line 77, in llm_generate
    response = llm_chain.invoke(input.query)
  File "/home/user/.local/lib/python3.11/site-packages/langchain/chains/base.py", line 163, in invoke
    raise e
  File "/home/user/.local/lib/python3.11/site-packages/langchain/chains/base.py", line 153, in invoke
    self._call(inputs, run_manager=run_manager)
  File "/home/user/.local/lib/python3.11/site-packages/langchain/chains/combine_documents/base.py", line 137, in _call
    output, extra_return_dict = self.combine_docs(
                                ^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/lib/python3.11/site-packages/langchain/chains/combine_documents/stuff.py", line 242, in combine_docs
    inputs = self._get_inputs(docs, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/lib/python3.11/site-packages/langchain/chains/combine_documents/stuff.py", line 198, in _get_inputs
    doc_strings = [format_document(doc, self.document_prompt) for doc in docs]
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/lib/python3.11/site-packages/langchain/chains/combine_documents/stuff.py", line 198, in <listcomp>
    doc_strings = [format_document(doc, self.document_prompt) for doc in docs]
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/lib/python3.11/site-packages/langchain_core/prompts/base.py", line 332, in format_document
    return prompt.format(**_get_document_info(doc, prompt))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/lib/python3.11/site-packages/langchain_core/prompts/base.py", line 284, in _get_document_info
    base_info = {"page_content": doc.page_content, **doc.metadata}
                                 ^^^^^^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'page_content'

Setting streaming to true works fine.