factor implemented as exponential might be incorrect

[jburroni]

currently, target += e is translated as:
pyro.sample(id, Exponential(1), -e).
This evaluate the log_pdf of the Exponential(1) at -e. If -e happens to be negative, a runtime check might fail.
A proper object for this task must be built.

[jburroni]

One solution is to build a class called Factor which might look like this:

class Factor(Distribution):
     def sample(self):
        assert(False)
     
    def log_prob(self, x):
        return x