Define API pdf sampling via e.g. probabilistic frameworks like edward

[lukasheinrich]

Description

We want to have the ability to sample from the pdf. A nice way to do this is via native probprog frameworks like Edward that hook in somewhat natively into tensor backends (not sure if there are similar projects for PyTorch, MXnet @cranmer ?). Not clear to me yet how to do this cleanly across numpy/TF/PyTorch/MXnet

For reference I added this super-simplified notebook to show how to sample sth like

p(n,a | alpha) = Pois(n |nu(alpha) ) * Gaus(a | alpha)

which is the core structure of HF right now

https://github.com/diana-hep/pyhf/blob/master/examples/experiments/edwardpyhf.ipynb

[lukasheinrich]

ok so I think pyro is the right analogue to edward for pytorch. MXnet has some distributions etc, but haven't found a 'framework' yes (but maybe that's not strictly needed

[matthewfeickert]

Closed through PR #558