Converts arrays to list.
        :param arr: numpy vector.
        :return: flattened list.

def _flatten(cls, arr) -> List[float]:
        """
        Converts arrays to list.
        :param arr: numpy vector.
        :return: flattened list.
        """
        if isinstance(arr, cls.SCALAR_ACTION_TYPES):
            arr = [float(arr)]
        if isinstance(arr, np.ndarray):
            arr = arr.tolist()
        if len(arr) == 0:
            return arr
        if isinstance(arr[0], np.ndarray):
            arr = [item for sublist in arr for item in sublist.tolist()]
        if isinstance(arr[0], list):
            arr = [item for sublist in arr for item in sublist]
        arr = [float(x) for x in arr]
        return arr