Merge pull request #541 from probcomp/fix-poisson-argument-grad

Fix `has_argument_grads` for Poisson distribution.

Author: ztangent

src/modeling_library/distributions/poisson.jl

@@ -24,6 +24,6 @@ end
 is_discrete(::Poisson) = true
 
 has_output_grad(::Poisson) = false
-has_argument_grads(::Poisson) = (false,)
+has_argument_grads(::Poisson) = (true,)
 
 export poisson