Parses out the Layer version from the arn

        Parameters
        ----------
        is_defined_within_template bool
            True if the resource is a Ref to a resource otherwise False
        arn str
            ARN of the Resource

        Returns
        -------
        int
            The Version of the LayerVersion

def _compute_layer_version(is_defined_within_template, arn):
        """
        Parses out the Layer version from the arn

        Parameters
        ----------
        is_defined_within_template bool
            True if the resource is a Ref to a resource otherwise False
        arn str
            ARN of the Resource

        Returns
        -------
        int
            The Version of the LayerVersion

        """

        if is_defined_within_template:
            return None

        try:
            _, layer_version = arn.rsplit(':', 1)
            layer_version = int(layer_version)
        except ValueError:
            raise InvalidLayerVersionArn(arn + " is an Invalid Layer Arn.")

        return layer_version