Expression to Pyro converter

[cthoyt]

@srtaheri @djinnome yesterday during the discussion about the LVM with do Calculus we realized that the paper wasn't actually implementing/applying the plug-in estimator using Stan, but rather was generating a SCM from the estimand coming from the ID algorithm. The new idea going forwards is to make a function to convert a given estimand into an AST of numpyro commands such that it can be simulated in pyro, given a prior distribution for each variable appearing in the estimand

PriorType = ...
PyroType = ...

def convert_pyro(expression: Expression, priors: Mapping[Variable, PriorType]) -> PyroType:
    ....