Fix/correct streaming resource lock

[gjpower]

Fixes #1861

Establishes the exit stack context inside the correct streaming response task so it may be closed correctly and avoids the issue of opening it in one task and trying to close it in a separate task.

Also simplifies usage of the exit_stack by using the AsyncContextManager directly async with contextlib.asynccontextmanager(...)()

does a basic refactor on create_completion and create_chat_completion moving repeated code to a separate function.

Adds a check for client disconnect before call to llm. This means that if a client creates a request but closes the connection before the server gets a lock on the llama_proxy to answer the request then the server will simply close the connection instead of calling the llm to generate the response for an already closed connection

[gjpower]

Some general clarifications about the scope of this pull request

[agronholm]

Not your fault @gjpower but the original code looks really weird. An async generator w/o the use of @asynccontextmanager? And manual use of acquire() and release()? What was the original author thinking?

[gjpower]

Not your fault @gjpower but the original code looks really weird. An async generator w/o the use of @asynccontextmanager? And manual use of acquire() and release()? What was the original author thinking?

@agronholm It looks like they were making use of the FastApi depends with generator feature that wraps the dependency resource with AsyncContextManager itself.
https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/
https://github.com/fastapi/fastapi/blob/0.89.1/fastapi/dependencies/utils.py#L463

If it wasn't for the streaming context the use of depends with a generator and a single lock would be sufficient. The explicit call to wrap with AsyncContextManager is required to allow it to work in the expected way with depends for the other endpoints.
Have a look at this discussion on FastAPI fastapi/fastapi#9054 (comment)

[abetlen]

@gjpower thank you, appreciate the fix!