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Truncation Sampling as Language Model Desmoothing

Posted on Edited on In Valine:

URL

TL;DR

  • 常用的 top-p random sampling decoding method 可能会导致输出的长文本质量较差
  • 本文设计了一种基于熵的动态概率阈值优化 top-p 随机采样算法，叫做 η−sampling\eta-sampling

Algorithm

公式表示

Ax<i={x∈V∣Pθ(x∣x<i)>η}A_{x \lt i}=\{x \in V | P_{\theta}(x | x \lt i) \gt \eta \}

η=min(ϵ,α(−hη,x<i))\eta=min(\epsilon,\alpha(-h_{\eta,x<i}))

其中第一行公式表示随机采样概率值截断到 η\eta

第二行中 ϵ,α\epsilon,\alpha 是超参数，通常 α=ϵ\alpha=\sqrt{\epsilon}，h 表示输出的熵，h=−(p∗log(p)).sum()h=-(p*log(p)).sum()，p 表示概率

代码表示

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class EtaWarper(transformers.LogitsWarper):
  """Our proposed eta sampling warper."""
  def __init__(self, epsilon):
    self.epsilon = epsilon
    self.filter_value = -float("Inf")

  def __call__(self, input_ids, scores) -> torch.FloatTensor:
    probabilities = scores.softmax(dim=-1)
    entropy = - (probabilities * torch.log(probabilities)).sum()
    eta = min(self.epsilon, torch.sqrt(torch.tensor(self.epsilon))*torch.exp(-entropy))
    indices_to_remove = probabilities < eta
    max_word = torch.argmax(scores,dim=-1)
    # 防止把候选词全删了，至少留一个
    indices_to_remove[...,max_word.squeeze()] = 0
    new_scores = scores.masked_fill(indices_to_remove, self.filter_value)
    return new_scores

Thought

  • 是 top-p 的小升级，计算量较小，算是一个小 trick