[rllib] Feature/soft actor critic v2 (#5328)

* Add base for Soft Actor-Critic

* Pick changes from old SAC branch

* Update sac.py

* First implementation of sac model

* Remove unnecessary SAC imports

* Prune unnecessary noise and exploration code

* Implement SAC model and use that in SAC policy

* runs but doesn't learn

* clear state

* fix batch size

* Add missing alpha grads and vars

* -200 by 2k timesteps

* doc

* lazy squash

* one file

* ignore tfp

* revert done

doc/source/rllib.rst

@@ -73,6 +73,8 @@ Algorithms
 
    -  `Proximal Policy Optimization (PPO) <rllib-algorithms.html#proximal-policy-optimization-ppo>`__
 
+   -  `Soft Actor Critic (SAC) <rllib-algorithms.html#soft-actor-critic-sac>`__
+
 *  Derivative-free
 
    -  `Augmented Random Search (ARS) <rllib-algorithms.html#augmented-random-search-ars>`__