Fix sac_agent debug summary bug w/ entropy.
Fix sac_agent debug summary bug w/ entropy.

SAC is usually used with a TransformedDistribution for the actor distribution, which does not have an analytic entropy.

PiperOrigin-RevId: 244279497
Change-Id: Ib595f02f98231b68f05c2a8cde5228c2f99186fb

tensorflow/agents,tensorflow/agents