Only update trainable (i.e. Mirrored) variables with strategy.extende.update.
Only update trainable (i.e. Mirrored) variables with strategy.extende.update.

PiperOrigin-RevId: 277088222
Change-Id: Ibe9fcc5d39f0788f6fb1cd6ac64e3221ed604eff

tensorflow/agents,tensorflow/agents