Hi!

I’m new to Pyro, and I really enjoy this framework. I would like to do some modelling that involves a multinomial distribution. I’ve read the enumeration tutorial, and I decided to try it:

```
def model():
multi = pyro.sample('multi', dist.Multinomial(2, torch.Tensor([1/N for i in range(N)])))
def guide():
multi = pyro.sample('multi', dist.Multinomial(2, torch.Tensor([1/N for i in range(N)])))
print(multi)
elbo = pyro.infer.TraceEnum_ELBO(max_plate_nesting=0)
elbo.loss(model, pyro.infer.config_enumerate(guide, "parallel"))
```

The result shows that there was no iteration done:

`tensor([0., 0., 0., 0., 1., 0., 0., 0., 0., 1.])`

I believe it’s possible to solve the same problem using 2 Categorical distributions, but I was wondering if you plan to support enumeration for multinomial distributions (or if you want to have it, maybe then I could put in my 2 cents).

Thanks!