Use multipledispatch for kl_divergence

[fehiepsi]

As @fritzo's suggestion in #748, we can use multipledispatch to simplify dispatching work in kl_divergence (and possibly in biject_to too?). We can do this when funsor becomes the main dependency of numpyro.

[fritzo]

For biject_to you could simply use Python 3's builtin singledispatch which requires no extra dependencies. We use that in Funsor for single dispatch. We had avoided that in PyTorch because PyTorch originally supported Python 2 which is missing singledispatch.

[fehiepsi]

Thanks for reviewing, @fritzo!