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arXiv 2306.01669cs.CVcs.LG

用CLIP增强CLIP:探索面向有限标签提示调优的伪标记方法

Enhancing CLIP with CLIP: Exploring Pseudolabeling for Limited-Label Prompt Tuning

Brown University(布朗大学)

Cristina Menghini, Andrew Delworth, Stephen H. Bach

AI总结:

针对CLIP等视觉语言模型下游微调标注数据有限的问题,提出基于零样本伪标签的统一提示调优框架,迭代优化伪标签可在多学习范式下显著提升分类准确率,且类别准确率分布更均衡。

摘要:

对CLIP这类视觉语言模型(VLM)进行下游任务微调通常是优化其性能的必要步骤,但一大障碍是标注数据的可用性有限。我们研究了伪标记(即为未标注数据生成启发式标签)通过提示调优增强CLIP的效果。传统伪标记方法先在标注数据上训练模型,再为未标注数据生成标签。而VLM的零样本能力催生了“第二代”伪标记方法,这类方法无需在标注数据上进行任务特定训练。我们发现,将零样本伪标签作为监督来源时,半监督学习、直推式零样本学习和无监督学习等学习范式都可被视为在优化同一个损失函数。这种统一视角有助于开发适用于多种学习范式的通用训练策略。我们在CLIP表现存在局限的图像分类任务上,通过改变提示模态(如文本提示或视觉提示)和学习范式对这些策略展开研究。我们发现:(1)此前未被探索的、迭代优化伪标签的提示调优策略能持续提升CLIP的准确率,在半监督学习中提升19.5个点,在直推式零样本学习中提升28.4个点,在无监督学习中提升15.2个点;(2)传统半监督伪标记会加剧模型对伪标签质量更高的类别的偏向,而提示调优则能让各类别准确率的分布更均衡。复现实验的代码可在https://github.com/BatsResearch/menghini-neurips23-code获取。

英文摘要:

Fine-tuning vision-language models (VLMs) like CLIP to downstream tasks is often necessary to optimize their performance. However, a major obstacle is the limited availability of labeled data. We study the use of pseudolabels, i.e., heuristic labels for unlabeled data, to enhance CLIP via prompt tuning. Conventional pseudolabeling trains a model on labeled data and then generates labels for unlabeled data. VLMs' zero-shot capabilities enable a "second generation" of pseudolabeling approaches that do not require task-specific training on labeled data. By using zero-shot pseudolabels as a source of supervision, we observe that learning paradigms such as semi-supervised, transductive zero-shot, and unsupervised learning can all be seen as optimizing the same loss function. This unified view enables the development of versatile training strategies that are applicable across learning paradigms. We investigate them on image classification tasks where CLIP exhibits limitations, by varying prompt modalities, e.g., textual or visual prompts, and learning paradigms. We find that (1) unexplored prompt tuning strategies that iteratively refine pseudolabels consistently improve CLIP accuracy, by 19.5 points in semi-supervised learning, by 28.4 points in transductive zero-shot learning, and by 15.2 points in unsupervised learning, and (2) unlike conventional semi-supervised pseudolabeling, which exacerbates model biases toward classes with higher-quality pseudolabels, prompt tuning leads to a more equitable distribution of per-class accuracy. The code to reproduce the experiments is at https://github.com/BatsResearch/menghini-neurips23-code.

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