[FR] make DiscreteHMCGibbs plate aware

[martinjankowiak]

afaik DiscreteHMCGibbs does not make use of plate information when computing Gibbs updates for discrete latent variables. it would be nice to support this, as leveraging this information can make Gibbs steps exponentially cheaper

[fehiepsi]

This sounds possible. I think we can perform discrete update in blocks (maybe #898 is helpful) where in each block, we can decide which strategy we want (i.e. using current DiscreteHMCGibbs strategy or plate-aware strategy). For plate-aware, I guess we just need to maintain plate-information of each factor, remove unnecessary factors (based on provenance?), then perform the gibbs update in one pass.