All `make_partials_propagator` to return its input args as a tuple

[SteveBronder]

When a user calls something like normal_lpdf(std::move(y), std::move(mu), std::move(sigma)) nothing really happens with the moved memory. But we could actually move those objects into the partials_propagator so that the partials_propagator does not have to make a hard copy of the inputs. A nice piece would be if make_partials_propagator returned a tuple of the partials propagator and references to the moved memory so that the functions would reuse that local memory.

auto [partials_prop, y_ref, mu_ref, sigma_ref] = make_partials_propagator(
  std::move(y), std::move(mu), std::move(sigma));

For autodiff types the inputs would be moved into the partials propagator and then references to the object would be the return. For non-autodiff types the object would just be passed back

This would replace a lot of the ref_type_t boilerplate inside of the distributions since the values would then sit inside of the partials propagator.