DistributionLambda for LikelihoodLayer

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The likelihood layer could be a DistributionLambda that returns the appropriate tfp distribution - in general, this would be a MixtureSameFamily of the tfp distribution describing the likelihood. This is closely related to GPflow/GPflow#1345 and might require some work/coordination with GPflow upstream. We'd like to keep the analytic variational expectations (part of GPflow, not part of tfp distributions). At a minimum, we could create an explicit mapping between gpflow likelihoods and tfp distributions.