Add allow_optional for Output

[T4rk1n]

Output should have allow_optional like the Input/State to not trigger the error message.

This could be useful with a devtool hook that place their component outside of the regular layout and are not expected to be in the layout if debug=False

[CNFeffery]

Is there a way to directly determine whether the current Dash application is in debug mode? The only method I know is to use werkzeug.serving.is_running_from_reloader() to check if the code execution is triggered by hot reloading, but this doesn't fully represent the debug mode.

[T4rk1n]

@CNFeffery The devtools are saved in app._dev_tools but they are not really set before the call to app.run.

You can get a reference to the app with hooks.setup and check it in either index, layout or callbacks.