Plated samples are incompatible with Pyro api

[fritzo]

One issue with funsor.minipyro is that the samples returned from a model are not compatible with tensor shapes and numpy shapes. This came up in pyro-ppl/pyro-api#5 as we try to create a generic API for multiple Pyro backends.

One solution might be to add a to_tensor() method for model results.

Another solution might be to start using named tensors or an align-like method.