Make Pyro more compatible with PyTorch idioms

[eb8680]

This issue tracks a few low-effort, backward-compatible improvements to the Pyro programming model that could be finished and released before the initial release of this package. See old version of changes in pyro-ppl/pyro#2996

Tasks:

• Add an option to stop PyroModules from sharing parameters pyro-ppl/pyro#3149 Add a torch.nn.Module interface to pyro.infer.ELBO classes
• Add an option to stop PyroModules from sharing parameters pyro-ppl/pyro#3149 Make Pyro parameters local to individual PyroModule instances
• Add conditional inverse and compose TransformModules pyro-ppl/pyro#3185 Move Transform utilities from Reproduce results in deep SCM tutorial #70 upstream

Stretch goals, not necessarily blocking a release:

• Eager PyroSample evaluation when PyroModule context is active pyro-ppl/pyro#3150 Support eager evaluation of PyroSample in PyroModule forward passes
• Add a higher-level syntax for reparameterization #56
• Have effect handlers applied to PyroModules return PyroModules
• Add initial evaluation of PyroSample and PyroParam attributes before forward evaluation
• Make query interfaces like pyro.condition more compatible with PyroModule name mangling

[SamWitty]

@eb8680 , can I try taking this on? I think this will push me to really get deep into Pyro and the reparameterization modules.

[eb8680]

Sure, although I think the remaining tasks are more involved and less urgent. I wanted to do the first three early so that I can do a Pyro minor release with the changes before this library goes public.