support: yield sse_starlette.ServerSentEvent in /stream

[itok01]

Allows the /stream endpoint to return sse_starlette.ServerSentEvent from runnable, allowing you to return custom events such as event: error.

The reason for not using langserve.server_sent_events.ServerSentEvent is that is unbable to check with isinstance() whether an object is a langserve.server_sent_events.ServerSentEvent.

Example

Run ./examples/custom_events/server.py

/stream:

$ curl -X POST -d '{"input":{},"config":{}}' 'http://127.0.0.1:8000/stream'
event: metadata
data: {"run_id": "40239ec7-e492-4c26-aac7-4380df1ad050"}

event: data
data: {"message":"Message 0"}

event: data
data: {"message":"Message 1"}

event: data
data: {"message":"Message 2"}

event: data
data: {"message":"Message 3"}

event: error
data: {"message":"An error occurred"}

event: end

/invoke:

$ curl -X POST -d '{"input":{},"config":{}}' 'http://127.0.0.1:8000/invoke'
{"output":{"message":"An error occurred"},"metadata":{"run_id":"06078459-1945-46d0-97ec-d7bec22f408a","feedback_tokens":[]}}

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