Silence tf.distributions deprecation messages caused by internal global function calls.

E.g. register_kl calls would trigger such warnings. This spam was exacerbated
by the fact that it happens before logging is initialized, so it is dumped
prominently to STDERR. Worse yet it also happened no matter whether the user
imported any symbols from tf.distributions or not as the relevant code is
executed when you import TensorFlow.

PiperOrigin-RevId: 216396036