def get_nested_dict_key(nested_dict):
    """Get all keys of nested dict

    Arguments
    ----------
    nested_dict : dict
        Nested dictionary

    Return
    ------
    all_nested_keys : list
        Key of nested dict
    """
    all_nested_keys = []
    for values in nested_dict.values():
        all_nested_keys += values.keys()
    return all_nested_keys
