Fix has_argument_grads for Poisson distribution.

Discovered in #540 (reply in thread). The implementation of `has_argument_grads` mistakenly claimed that the gradient wasn't implemented for the first argument of the Poisson distribution.

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