概念组合性:解读基础模型的新视角

近年来,基础模型在各个领域都取得了令人瞩目的成就,然而,其黑箱特性也为调试、监控、控制和信任这些模型带来了巨大挑战。概念解释作为一种新兴方法,试图利用诸如物体属性(例如条纹)或语言情感(例如快乐)等单个概念来解释模型的行为。通过将模型学习到的表示分解为多个概念向量,可以推导出这些概念。例如,模型对一张狗的图像的嵌入可以分解为代表其毛发、鼻子和尾巴的概念向量的总和。

现有方法的不足

现有的基于 PCA 或 KMeans 等方法的工作能够很好地提取基本概念的向量表示。例如,图 1 展示了从 CLIP 模型中提取的 CUB 数据集中的图像,这些图像包含了 PCA 学习到的概念。这些技术能够正确地提取诸如“白色鸟类”和“小型鸟类”等概念的表示,然而,将它们的表示相加并不能得到“小型白色鸟类”这一概念的表示。

概念组合性的重要性

概念的组合性对于以下几个用例至关重要:

  • 模型预测解释: 通过组合概念来解释模型预测。
  • 模型行为编辑: 组合性概念允许编辑细粒度的模型行为,例如在不影响其他行为的情况下提高大型语言模型的真实性。
  • 新任务训练: 可以训练模型组合基本概念来完成新任务,例如使用喙的形状、翅膀的颜色和环境等概念对鸟类进行分类。

概念组合性的评估

为了评估概念组合性,我们首先在受控环境下验证了概念的真实表示的组合性。我们观察到,概念可以被分组为属性,其中每个属性都包含关于某些共同属性的概念,例如物体的颜色或形状。来自不同属性的概念(例如蓝色和立方体)可以组合,而来自同一属性的概念(例如红色和绿色)则不能。我们还观察到,来自不同属性的概念大致正交,而来自同一属性的概念则不然。

概念组合性提取 (CCE)

为了提取组合性概念,我们提出了 CCE 方法。该方法的关键思想是一次性搜索整个概念子空间,而不是单个概念,从而允许 CCE 强制执行上述组合性概念的属性。CCE 算法主要包含以下步骤:

  1. 学习子空间 (LearnSubspace): 优化一个子空间,使得该子空间中的数据能够根据固定的聚类中心进行良好的聚类。
  2. 学习概念 (LearnConcepts): 在学习到的子空间中执行球形 K 均值聚类,以识别概念。
  3. 迭代优化: 交替执行学习子空间和学习概念步骤,直到收敛。

实验结果

我们在视觉和语言数据集上进行了广泛的实验,结果表明:

  • 在受控环境下,CCE 比现有方法更能有效地组合概念。
  • 在真实数据环境下,CCE 能够成功地发现新的、有意义的组合性概念。
  • CCE 提取的组合性概念可以提高下游任务的性能。

结论

本文从组合性的角度研究了基础模型的概念解释。我们验证了从这些模型中提取的真实概念是组合性的,而现有的无监督概念提取方法通常不能保证组合性。为了解决这个问题,我们首先确定了组合性概念表示的两个显著属性,并设计了一种新的概念提取方法 CCE,该方法在设计上尊重这些属性。通过对视觉和语言数据集进行的大量实验,我们证明了 CCE 不仅可以学习组合性概念,还可以提高下游任务的性能。

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