Make Lattice work with partition functions

[mofeing]

Depends on #241

Partition functions don't have open indices, nor input/output. But the way we have designed the site system in Quantum is that Sites map to open-indices, so something like a partition function has no sites. This is not really a problem because Quantum was designed for connectivity in mind but it's also the reason why we postposed partition functions support in Tenet.

Now that we have Lattice and Ansatz, we should be able to support them. The only thing left is that we might require to map Sites to Tensors in Ansatz. Besides, because Tensors in a TensorNetwork are contracted between them at some steps of some algorithms, we should have a way to keep track and deambiguate at these cases; e.g. 2 Sites will map to the same Tensor. Due to that, we should first resolve #241.