Add automatic dependency tracking to AutoStructured guide

[fritzo]

Addresses #2813
get_dependencies() was pair coded with @eb8680

This adds a helper get_dependencies() and uses that to automatically create a sparse dependency graph in the AutoStructured guide.

Tasks

• Implement a get_dependencies() helper
• Integrate automatic guide structure in AutoStructured
• Convert prior structure to posterior structure as in (Webb et al. 2018)

Tested

• tests for get_dependencies()
• smoke tests for AutoStructured with default params
• test of posterior guide structure

[fritzo]

@eb8680 ready for a second review.

[eb8680]

Looks good, just a few nits

[fritzo]

Another motivating paper:
Manfred Opper, C´edric Archambeau (2009)
"The Variational Gaussian Approximation Revisited"
http://www0.cs.ucl.ac.uk/staff/c.archambeau/publ/neco_mo09_web.pdf

[eb8680]

Looks good!