Implement AutoWrap and AutoWrapFull guides

[fritzo]

Addresses #2813

This implements a new method of constructing autoguides, AutoWrap, that allow guides to leverage deterministic computations in the model, e.g. those introduced by reparameterization.

Tasks

• mask model sites in the guide?
• return deterministic sites in get_dependencies()
• trace all sites in ._setup_prototype()
• replace matrix-vector ops with einsum ops
• avoid re-running the model in SVI
• avoid re-running the model in Predictive

Tested

• add to test_autoguide.py
• test structure inference
• test on a real world model

[fritzo]

Closing in favor of #2929