PR author/subcription/notification

[bhack]

We are still having the same well known issues with copy-bara bot PRs.

See our full thread with @qlzh727 at Keras keras-team/keras#17183 (comment)

[bhack]

To summarize:

1. we have commit done by non Github users
2. we PRs submitted by bot without having the Github author subscribed/assigned for comment/review
3. we have case where the first two point are present at the same time in the same PR

[bhack]

This is a quite common issue across repositories with Google copybara tools.

I hope that we could have a common effort to resolve this, with a standardized process, especially if one of this day (I hope) we want to introduce this process also in the TF org repos (/cc @MichaelHudgins)