More DistributionLambda in layers

[vdutor]

• LikelihoodLayer could return the correctly parametrized y-distribution (perhaps a mixture if using multiple samples?)
• LatentVariableLayer could be a tfp.DistributionLambda and return prior/posterior as appropriate (we'd then need to take out again the composition with inputs through addition or concatenation).
• Encoder could also return the distribution directly instead of keeping track of the class of the prior.

[st--]

Split this up into #7, #8, and #9.