API: Use FastAPI streaming instead of sse_starlette

sse_starlette kept firing a ping response if it was taking too long
to set an event. Rather than using a hacky workaround, switch to
FastAPI's inbuilt streaming response and construct SSE requests with
a utility function.

This helps the API become more robust and removes an extra requirement.

Signed-off-by: kingbri <bdashore3@proton.me>

model.py

@@ -311,6 +311,7 @@ class ModelContainer:
         stop_conditions: List[Union[str, int]] = kwargs.get("stop", [])
         ban_eos_token = kwargs.get("ban_eos_token", False)
 
+
         # Ban the EOS token if specified. If not, append to stop conditions as well.
 
         if ban_eos_token: