Fix AutoGaussian and tests

[fritzo]

Addresses #2813
Blocking #2946
Blocked by #2952

This fixes a number of bugs in AugoGaussian and two bugs in tests that were silencing the bugs in AutoGaussian.

Fixes & tests

• Fixed a bug in backend dispatch and added a regression test test_backend_dispatch
• Fixed tests.autoguide.test_gaussian.check_structure().
  Hand mutation-tested that check_structure() is now strict.
  Fixed a number of expected sparsity structure matrices and manually verified changes.
• Fixed prototype_trace missing observation nodes and added regression tests checking guide.dependencies
• Fixed numerical instability around the initial point and and resurrected pyrocov_model_relaxed from Update to use funsor's SRIF Gaussian #2943 which had been failing due to singular gradient.

Remaining errors

• incorrect posterior variance in funsor backend
  pytest tests/infer/test_autoguide.py -k exact --stage=funsor
• bad ELBO gradients in funsor backend
  pytest tests/infer/autoguide/test_gaussian.py -k structure

[eb8680]

Looks good per zoom review

[fritzo]

Thanks for reviewing @eb8680 !