Support streaming text responses from functions

[alexellis]

Description

Support streaming responses from functions

Motivation and Context

When responses are written back piece-by-piece through Server Sent Events, the gateway was buffering all of the text then sending it at once, or missing some data. This change implements custom handling for the content-type of text/event-stream

How Has This Been Tested?

from flask import stream_with_context, request,Response
import requests

from langchain_community.chat_models import ChatOpenAI
from os import environ

environ["OPENAI_API_KEY"] = "Bearer foo"

chat_model = ChatOpenAI(
    model="gpt-3.5-turbo",
    openai_api_base="https://openai.inlets.dev/v1",
)

def handle(app, req):
    def stream():
        for chunk in chat_model.stream("You are a helpful AI assistant, try your best to help, respond with truthful answers, but if you don't know the correct answer, say so. Answer this question: What are the top 5 countries by GDP?"):
            print(chunk.content+"\n", flush=True, end="")
            yield f'data: {chunk.content}\n\n'

    return Response(stream(), mimetype='text/event-stream')

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)