Fix get_dependencies(), add poutine.get_plates()

[fritzo]

Blocking #2929

This PR adds some fixes and machinery needed for #2929, aiming to simplify that growing PR.

• makes a number of fixes to get_inspect(), adding regression tests:
  • support for pyro.factor() statements
  • support for discrete variables
  • support for use inside torch.no_grad(), as in autoguide init
• adds a poutine.get_plates() and adds unit tests (also adds tests for poutine.get_mask())
• fixes a typo in the Logistic distribution
• works around coveralls flakiness

[eb8680]

Looks good