Add stop_gradient to the td_targets in SAC actor_loss.
Add stop_gradient to the td_targets in SAC actor_loss.

This prevents gradients in shared layers between actor/critic networks to flow
through the critic when calculating the actor_loss.

PiperOrigin-RevId: 309801742
Change-Id: I9114abf5016afcfc3b5de16dd13fdebdb894947c

tensorflow/agents,tensorflow/agents