Add HMC and NUTS algorithms / interpretations

[fritzo]

Blocked by Pyro HMC refactoring https://github.com/pyro-ppl/pyro/issues/1816

Initially we can simply wrap Pyro's HMC and NUTS implementations. After NumPyro's distributions are closer to torch.distributions and after Funsor's Numpy support improves, we can also implement a NumPyro-based HMC and NUTS.

Tasks

• [ ] Add shim for Pyro's HMC and NUTS
• [ ] Add an examples/nuts.py

[fritzo]

cc @neerajprad who is refactoring Pyro's HMC and NUTS to support this