Gibbs sampling for HMM

[slinderman]

HMMs with exponential family emissions admit a simple Gibbs sampling algorithm: alternate between the following two steps:

1. Sample the discrete latent states given the parameters and data using HMMPosterior.sample()
2. Sample the parameters from their conditional distribution given the latent states and data. This will follow the same recipe as the ExponentialFamilyEmissions.m_step(), but it will use conditional.sample() instead of conditional.mode().