an API ?

React native is a js vm + a bridge + native rendering. they are not related at all. so unless you write a python to JS parser this unlikely to happen on the phone.

go for the API which means you are frontend agnostic but there is no offline ML.

I'm assuming here that you don't mean "running python in the device the app is targeted to alongside react" because that's a can of worms all by itself.

If you meant "integrate it with a server running python" then it's no different than integrating it with any other server. Like j said, just create an API, in python, your app can communicate with and you're done.