NCL: Textual Backdoor Defense Using Noise-Augmented Contrastive Learning - Shengfang Zhai; Qingni Shen; Xiaoyi Chen; Weilong Wang; Cong Li; Yuejian Fang; Wu, Zhonghai (IEEE International Conference on Acoustics, Speech and Signal Processing ICASSP 2023, Rhodes Island, Greece, June 4-10, 2023, 2023)