[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-env.rst

@@ -18,6 +18,7 @@ DQN, Rainbow    **Yes** `+parametric`_  No                  **Yes**      No
 DDPG, TD3       No                      **Yes**             **Yes**      No
 APEX-DQN        **Yes** `+parametric`_  No                  **Yes**      No
 APEX-DDPG       No                      **Yes**             **Yes**      No
+SAC             (todo)                  **Yes**             **Yes**      No
 ES              **Yes**                 **Yes**             No           No
 ARS             **Yes**                 **Yes**             No           No
 QMIX            **Yes**                 No                  **Yes**      **Yes**