check whether the file of customized tuner/assessor/advisor exists
    spec_key: 'tuner', 'assessor', 'advisor'

def validate_customized_file(experiment_config, spec_key):
    '''
    check whether the file of customized tuner/assessor/advisor exists
    spec_key: 'tuner', 'assessor', 'advisor'
    '''
    if experiment_config[spec_key].get('codeDir') and \
        experiment_config[spec_key].get('classFileName') and \
        experiment_config[spec_key].get('className'):
        if not os.path.exists(os.path.join(
                experiment_config[spec_key]['codeDir'],
                experiment_config[spec_key]['classFileName'])):
            print_error('%s file directory is not valid!'%(spec_key))
            exit(1)
    else:
        print_error('%s file directory is not valid!'%(spec_key))
        exit(1)