Reorganize constraints and transforms modules

[fritzo]

Addresses issues in #2049

This reorganizes the constraints and transforms modules such that:

• pyro.distributions strictly extends torch.distributions
• pyro.distributions.constraints strictly extends torch.distributions.constaints
• pyro.distributions.transforms strictly extends torch.distributions.transforms

This is important for pyro.generic so all constraints and transforms are accessible, both those from PyTorch and those from Pyro.

After this PR we can idiomatically:

import pyro.distributions as dist
from pyro.distributions import constraints, transforms

or more generically, following #2049 ,

from pyro.generic import distributions as dist
from pyro.generic import constraints, transforms

cc @stefanwebb

[neerajprad]

Looks great! Thanks for refactoring this. Will merge when tests pass.