Add support for numpyro, jax to backends

numpyro : http://pyro.ai/examples/intro_long.html jax : https://jax.readthedocs.io/en/latest/jax.html

In particular, numpyro (which relies on jax) should provide a numpy-like API that will then support the autodifferentiation and just-in-time (JIT) compilation needed for things like HMC (NUTs) samplers, etc.