image: Bring back concurrency limit.

app.py

@@ -74,7 +74,10 @@ with gr.Blocks(theme=gr.themes.Soft()) as app:
     gr.Examples(EXAMPLES, inputs=[model, size, input], outputs=[image],
         fn=playground, cache_examples=False, run_on_click=True)
 
-app.queue(max_size=3).launch(
+app.queue(
+    max_size=3,
+    default_concurrency_limit=3
+).launch(
     show_api=False,
     server_name="0.0.0.0",
     pwa=True