def __init__(self, prior_sigma=None, prior_strength=1.0):
        """
        Half-normal distribution.

        https://en.wikipedia.org/wiki/Half-normal_distribution

        - Univariate continuous distribution.
        - Support: :math:`\\mathbb{R}_{>0}`.
        - Parameter: rate :math:`\\sigma > 0`.

        Parameters
        ----------
        prior_sigma
            standard deviation :math:`\\sigma` of the prior half-normal distribution (default: None).
        prior_strength
            strength of the loss regularisation induced by the prior
        """
        self.prior_sigma = prior_sigma
        _check_strict_positive(self.prior_sigma, "sigma")
        self.softplus = nn.Softplus()
        super().__init__(
            likelihood_type=LikelihoodType.HalfNormal,
            parameter_names=["sigma"],
            prior_strength=prior_strength,
        )

        Half-normal distribution.

        https://en.wikipedia.org/wiki/Half-normal_distribution

        - Univariate continuous distribution.
        - Support: :math:`\mathbb{R}_{>0}`.
        - Parameter: rate :math:`\sigma > 0`.

        Parameters
        ----------
        prior_sigma
            standard deviation :math:`\sigma` of the prior half-normal distribution (default: None).
        prior_strength
            strength of the loss regularisation induced by the prior
        
darts/utils/likelihood_models/torch.py