[TF-Agents] Avoid setting tf.config.set_device_placement if it's already set.
[TF-Agents] Avoid setting tf.config.set_device_placement if it's already set.

This speeds up dqn_train_eval from ~470 steps/sec to ~500 steps/sec, giving
a 6% performance boost.

Fixes #569; though other changes by TF team address the actual memory
leak (see https://github.com/tensorflow/tensorflow/issues/48888).

PiperOrigin-RevId: 372476956
Change-Id: Ifb1f772d31e8b2194ba38cde565730b5b6cc08c2

tensorflow/agents,tensorflow/agents