Move the `DefaultTransform` logic to AeMCMC

[rlouf]

The transformations that should be applied are sampler-specific, so automation should not happen in AePPL.

[brandonwillard]

Is the basic idea that we should generate log-probabilities with AePPL and apply the transforms to those graphs using AeMCMC, instead of doing all that together in AePPL?

[rlouf]

Yes. AeMCMC would transform the graph and specify the variables to condition on. But that might only make sense when AePPL is able to condition on transformed variables e.g. at.exp(x_rv) directly, which it should be able to do regardless.

It would make sense to eventually have a separate library that contains the rewrite engine based on miniKanren and the mathematical relations that both AePPL and AeMCMC need to respectively build a logprob graph and build a sampler graph.