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（图形嘶嘶作响）
(graphics whooshing)

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- 如何预处理句子对。
- How to pre-process pairs of sentences.

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我们已经看到了如何分词化单个句子
We have seen how to tokenize single sentences

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并将它们放在一起，
and batch them together in the

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在 “Batching inputs together” 视频中
"Batching inputs together" video.

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如果这段代码对你而言不熟悉，
If this code look unfamiliar to you,

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请务必再次查看该视频。
be sure to check that video again.

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这里将重点关注对句子进行分类的任务。
Here will focus on tasks that classify pair of sentences.

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例如，我们可能想要对两个文本是否被释义
For instance, we may want to classify whether two texts

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进行分类。
are paraphrased or not.

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这是从 Quora Question Pairs 数据集中获取的示例
Here is an example taken from the Quora Question Pairs dataset

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它专注于识别重复的问题。
which focuses on identifying duplicate questions.

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在第一对中，两个问题是重复的，
In the first pair, the two questions are duplicates,

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在第二个它们不是。
in the second they are not.

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另一个对分类问题是
Another pair classification problem is

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当我们想知道两个句子是
when we want to know if two sentences are

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逻辑上相关与否，
logically related or not,

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一个称为自然语言推理或 NLI 的问题。
a problem called natural language inference or NLI.

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在这个取自 MultiNLI 数据集的例子中，
In this example, taken from the MultiNLI data set,

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对于每个可能的标签，我们都有一对句子。
we have a pair of sentences for each possible label.

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矛盾，自然的或蕴涵，
Contradiction, natural or entailment,

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这是第一句话的奇特表达方式
which is a fancy way of saying the first sentence

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暗示第二种。
implies the second.

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所以分类成对的句子是一个
So classifying pairs of sentences is a problem

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值得研究的问题。
worth studying.

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事实上，在 GLUE 基准中，
In fact, in the GLUE benchmark,

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这是文本分类问题的学术界基准
which is an academic benchmark for text classification

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10 个数据集中有 8 个是关注
8 of the 10 datasets are focused

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使用句子对的任务。
on tasks using pairs of sentences.

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这就是为什么像 BERT 这样的模型
That's why models like BERT

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通常预先训练有双重目标。
are often pre-trained with a dual objective.

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在语言建模目标之上，
On top of the language modeling objective,

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他们通常有一个与句子对相关的目标。
they often have an objective related to sentence pairs.

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例如，在预训练期间 BERT 见到
For instance, during pretraining BERT is shown

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成对的句子，必须同时预测
pairs of sentences and must predict both

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随机掩蔽的标记值，以及第二个是否
the value of randomly masked tokens, and whether the second

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句子是否接着第一个句子。
sentence follow from the first or not.

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幸运的是，来自 Transformers 库的分词器
Fortunately, the tokenizer from the Transformers library

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有一个很好的 API 来处理成对的句子。
has a nice API to deal with pairs of sentences.

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你只需要将它们作为两个参数传递
You just have to pass them as two arguments

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到 tokenizer 。
to the tokenizer.

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在我们已经研究过的输入 ID 
On top of the input IDs and the attention mask

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和注意掩码之上，
we studied already,

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它返回一个名为标记类型 ID 的新字段，
it returns a new field called token type IDs,

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它告诉模型哪些标记属于
which tells the model which tokens belong

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第一句话，
to the first sentence,

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哪些属于第二句。
and which ones belong to the second sentence.

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放大一点，这里有一个输入 ID
Zooming in a little bit, here has an input IDs

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与它们对应的 token 对齐，
aligned with the tokens they correspond to,

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它们各自的标记类型 ID 和注意掩码。
their respective token type ID and attention mask.

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我们可以看到分词器还添加了特殊标记。
We can see the tokenizer also added special tokens.

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所以我们有一个 CLS token ，来自第一句话的 token ，
So we have a CLS token, the tokens from the first sentence,

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一个 SEP 标记，第二句话中的标记，
a SEP token, the tokens from the second sentence,

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和最终的 SEP 标记。
and a final SEP token.

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如果我们有几对句子，
If we have several pairs of sentences,

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我们可以通过第一句话的传递列表
we can tokenize them together by passing the list

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将它们标记在一起，然后是第二句话的列表
of first sentences, then the list of second sentences

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以及我们已经研究过的所有关键字参数
and all the keyword arguments we studied already

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例如 padding=True。
like padding=True.

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放大结果，
Zooming in at the result,

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我们可以看到分词器如何添加填充
we can see how the tokenizer added padding

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到第二对句子使得两个输出的
to the second pair sentences to make the two outputs

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长度相同，并正确处理标记类型 ID
the same length, and properly dealt with token type IDs

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和两个句子的注意力掩码。
and attention masks for the two sentences.

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然后就可以通过我们的模型了。
This is then all ready to pass through our model.