def METHOD_NAME(
    self,
    sampler: "sampler.MetropolisSampler",  # noqa: F821
    machine: nn.Module,
    params: PyTree,
    key: PRNGKeyT,
) -> Optional[Any]:
    """
    Initialises the optional internal state of the Metropolis sampler transition
    rule.
    The provided key is unique and does not need to be split.
    It should return an immutable data structure.
    Arguments:
        sampler: The Metropolis sampler.
        machine: A Flax module with the forward pass of the log-pdf.
        params: The PyTree of parameters of the model.
        key: A Jax PRNGKey.
    Returns:
        An optional state.
    """
    return None