Issue reproducing the example in Readme  #65

from sglang import function, system, user, assistant, gen, set_default_backend, OpenAI

@function
def multi_turn_question(s, question_1, question_2):
    s += system("You are a helpful assistant.")
    s += user(question_1)
    s += assistant(gen("answer_1", max_tokens=256))
    s += user(question_2)
    s += assistant(gen("answer_2", max_tokens=256))

set_default_backend(OpenAI("gpt-3.5-turbo"))

state = multi_turn_question.run(
    question_1="What is the capital of the United States?",
    question_2="List two local attractions.",
)

for m in state.messages():
    print(m["role"], ":", m["content"])

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
[<ipython-input-3-ac9af75fa374>](https://localhost:8080/#) in <cell line: 11>()
      9     s += assistant(gen("answer_2", max_tokens=256))
     10 
---> 11 set_default_backend(OpenAI("gpt-3.5-turbo"))
     12 
     13 state = multi_turn_question.run(

[/usr/local/lib/python3.10/dist-packages/sglang/backend/openai.py](https://localhost:8080/#) in __init__(self, model_name, *args, **kwargs)
     52     def __init__(self, model_name, *args, **kwargs):
     53         super().__init__()
---> 54         self.client = openai.OpenAI(*args, **kwargs)
     55 
     56         if isinstance(openai, Exception):

AttributeError: 'ModuleNotFoundError' object has no attribute 'OpenAI'