Remove last_permutation from SwapMCState initialization

[curtischong]

When doing a MC swap, we need to pass in last_permutation. But I think the SwapMCState can just figure out an initial last_permutation automatically (no need to pass it into the state at first?). Is there a strong reason why we need to pass it in?

@dataclass(kw_only=True)
class SwapMCState(SimState):
    """State for Monte Carlo simulations with swap moves.

    This class extends the SimState to include properties specific to Monte Carlo
    simulations, such as the system energy and records of permutations applied
    during the simulation.

    Attributes:
        energy (torch.Tensor): Energy of the system with shape [batch_size]
        last_permutation (torch.Tensor): Last permutation applied to the system,
            with shape [n_atoms], tracking the moves made for analysis or reversal
    """

    energy: torch.Tensor
    last_permutation: torch.Tensor

    _atom_attributes = SimState._atom_attributes | {"last_permutation"}  # noqa: SLF001
    _system_attributes = SimState._system_attributes | {"energy"}  # noqa: SLF001

Maybe a better approach is to make it | None so people don't need to pass it in when they create this swap state?

[orionarcher]

Yeah that makes sense, we can modify the __post_init__ to add in a default last swap.