Add TMC estimator of reweighted wake-sleep wake-phi loss

[eb8680]

Tasks:

• Add example
• Reuse traces across wake-theta and wake-phi loss computations
• Attempt to remove zero-expectation terms, similar to pyro.infer.Dice.compute_expectation
• Add stronger tests and tests for new ops.packed utilities
• Explore example performance for fixed model as function of noise and length scale for various learning settings
• Compute KL(P|Q) exactly with Gaussian math
• Evaluate posterior mean RMSE