Momentum update is never used in the code

[datonefaridze]

Hi, I was going thought the code and couldn't find where momentum encoder was being updated, I think it is initialized only once at the beginning and then isn't trained at all

[Chengyui]

Cause the CURL encoder is a part of the critic and critic_target. In SAC, critic and critic_target need to be soft_update. So there is no need to do it again in CURL.