How to use the fvcore.common.config.CfgNode function in fvcore

def gen_default_cfg():
        cfg = CfgNode()
        cfg.KEY1 = "default"
        cfg.KEY2 = "default"
        cfg.EXPRESSION = [3.0]

        return cfg

# -*- coding: utf-8 -*-
# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved

import logging
from fvcore.common.config import CfgNode as _CfgNode


class CfgNode(_CfgNode):
    """
    The same as `fvcore.common.config.CfgNode`, but different in:

    1. Use unsafe yaml loading by default.
      Note that this may lead to arbitrary code execution: you must not
      load a config file from untrusted sources before manually inspecting
      the content of the file.
    2. Support config versioning.
      When attempting to merge an old config, it will convert the old config automatically.
    """

    # Note that the default value of allow_unsafe is changed to True
    def merge_from_file(self, cfg_filename: str, allow_unsafe: bool = True) -> None:
        loaded_cfg = _CfgNode.load_yaml_with_base(cfg_filename, allow_unsafe=allow_unsafe)
        loaded_cfg = type(self)(loaded_cfg)