feat(router): add header option to disable buffering for the generate_stream response (#498)

# This PR adds an http header option to disable buffering for the
generate_stream endpoint response stream.

Problem: If a model is run behind a proxy server such as nginx that has
buffering enabled then the response stream from generate_stream gets
aggregated into a single response which basically disables streaming.
Instead of getting a chunked response where each token is presented over
time the response presents everything all at once.

Solution: This change adds the `X-Accel-Buffering` http header which
disables buffering for the generate_stream response, allowing the
response to stream properly.

router/src/server.rs

@@ -351,6 +351,7 @@ async fn generate_stream(
         "x-compute-characters",
         compute_characters.to_string().parse().unwrap(),
     );
+    headers.insert("X-Accel-Buffering", "no".parse().unwrap());
 
     let stream = async_stream::stream! {
         // Inference