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（微妙的爆炸）
(subtle blast)

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- 管道函数内部发生了什么？
- What happens inside the pipeline function?

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在这段视频中，我们将看看实际发生了什么
In this video, we will look at what actually happens

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当我们使用 Transformers 库的
when we use the pipeline function

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pipeline 函数时
of the Transformers library.

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更具体地说，我们将看看
More specifically, we will look at

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情绪分析管道，
the sentiment analysis pipeline,

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以及它是如何从以下两个句子中得出的
and how it went from the two following sentences

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正负标签
to the positive and negative labels

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加上各自的分数。
with their respective scores.

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正如我们在管道视频中看到的那样，
As we have seen in the pipeline video,

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管道分为三个阶段。
there are three stages in the pipeline.

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首先，我们将原始文本转换为数字
First, we convert the raw texts to numbers

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该模型可以使用分词器来理解。
the model can make sense of, using a tokenizer.

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然后，这些数字通过模型，
Then, those numbers go through the model,

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输出 logits 。
*[译者注: logits 作为逻辑值的意思]
which outputs logits.

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最后是后处理步骤
Finally, the post-processing steps

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将这些 logits 转换为标签和分数。
transforms those logits into labels and score.

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让我们详细看看这三个步骤，
Let's look in detail at those three steps,

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以及如何使用 Transformers 库复现它们，
and how to replicate them using the Transformers library,

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从第一阶段开始，分词化。
beginning with the first stage, tokenization.

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分词化过程有几个步骤。
The tokenization process has several steps.

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首先，文本被分成称为 token 的小块。
First, the text is split into small chunks called token.

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它们可以是单词、单词的一部分或标点符号。
They can be words, parts of words or punctuation symbols.

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然后分词器将有一些特殊的 token 
Then the tokenizer will had some special tokens

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如果模型期望它们。
if the model expect them.

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在这里，所使用的模型在开头需要一个 CLS token 
Here, the model used expects a CLS token at the beginning

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以及用于分类的句子末尾的 SEP token。
and a SEP token at the end of the sentence to classify.

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最后，标记器将每个标记与其唯一 ID 匹配
Lastly, the tokenizer matches each token to its unique ID

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在预训练模型的词汇表中。
in the vocabulary of the pretrained model.

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要加载这样的分词器，
To load such a tokenizer,

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Transformers 库提供了 AutoTokenizer API。
the Transformers library provides the AutoTokenizer API.

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这个类最重要的方法是 from_pretrained，
The most important method of this class is from_pretrained,

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这将下载并缓存配置
which will download and cache the configuration

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以及与给定 checkpoint 相关联的词汇表。
*[译者注: 在深度学习中, checkpoint 作为检查点是用来备份模型的, 后不翻译]
and the vocabulary associated to a given checkpoint.

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这里默认使用的 checkpoint
Here, the checkpoint used by default

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用于情绪分析的 pipeline
for the sentiment analysis pipeline

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是 distilbert-base-uncased-finetuned-sst2-English，
is distilbert-base-uncased-finetuned-sst2-English,

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这有点含糊。
which is a bit of a mouthful.

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我们实例化一个与该 checkpoint 关联的分词器，
We instantiate a tokenizer associated with that checkpoint,

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然后给它输入两个句子。
then feed it the two sentences.

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由于这两个句子的大小不同，
Since those two sentences are not of the same size,

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我们需要填充最短的一个
we will need to pad the shortest one

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能够构建一个数组。
to be able to build an array.

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这是由分词器完成的
This is done by the tokenizer

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使用选项 padding=True。
with the option padding=True.

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使用 truncation=True，我们确保任何句子更长
With truncation=True, we ensure that any sentence longer

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比模型可以处理的最大值被截断。
than the maximum the model can handle is truncated.

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最后，return_tensors 选项告诉分词器
Lastly, the return_tensors option tells the tokenizer

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返回 PyTorch 张量。
to return a PyTorch tensor.

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看看结果，
Looking at the result,

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我们看到我们有一个有两个键的字典。
we see we have a dictionary with two keys.

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输入 ID 包含两个句子的 ID，
Input IDs contains the IDs of both sentences,

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在应用填充的地方使用零。
with zeros where the padding is applied.

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第二把钥匙，注意力掩码，
The second key, attention mask,

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指示已应用填充的位置，
indicates where padding has been applied,

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所以模型不会关注它。
so the model does not pay attention to it.

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这就是标记化步骤中的全部内容。
This is all what is inside the tokenization step.

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现在让我们来看看第二步，模型。
Now let's have a look at the second step, the model.

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至于分词器，有一个 AutoModel API，
As for the tokenizer, there is an AutoModel API,

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使用 from_pretrained 方法。
with a from_pretrained method.

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它将下载并缓存模型的配置
It will download and cache the configuration of the model

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以及预训练的权重。
as well as the pretrained weights.

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但是，AutoModel API 只会实例化
However, the AutoModel API will only instantiate

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模型的主体，
the body of the model,

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也就是说，模型中剩下的部分
that is, the part of the model that is left

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一旦预训练头被移除。
once the pretraining head is removed.

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它会输出一个高维张量
It will output a high-dimensional tensor

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这是通过的句子的表示，
that is a representation of the sentences passed,

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但这不是直接有用的
but which is not directly useful

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对于我们的分类问题。
for our classification problem.

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这里张量有两个句子，
Here the tensor has two sentences,

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每十六个 token，
each of sixteen token,

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最后一个维度是我们模型的隐藏大小，768。
and the last dimension is the hidden size of our model, 768.

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要获得与我们的分类问题相关的输出，
To get an output linked to our classification problem,

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我们需要使用 AutoModelForSequenceClassification 类。
we need to use the AutoModelForSequenceClassification class.

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它与 AutoModel 类完全一样工作，
It works exactly as the AutoModel class,

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除了它会建立一个模型
except that it will build a model

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带分类头。
with a classification head.

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每个常见的 NLP 任务在 Transformers 库
There is one auto class for each common NLP task

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都有一个自动类
in the Transformers library.

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在这里，在给我们的模型两个句子之后，
Here, after giving our model the two sentences,

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我们得到一个大小为二乘二的张量；
we get a tensor of size two by two;

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每个句子和每个可能的标签都有一个结果。
one result for each sentence and for each possible label.

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这些输出还不是概率。
Those outputs are not probabilities yet.

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我们可以看到它们的总和不为 1。
We can see they don't sum to 1.

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这是因为 Transformers 库的每个模型
This is because each model of the Transformers library

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返回 logits 。
returns logits.

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为了理解这些 logits ，
To make sense of those logits,

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我们需要深入研究第三个
we need to dig into the third

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管道的最后一步，后处理。
and last step of the pipeline, post-processing.

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要将 logits 转换为概率，
To convert logits into probabilities,

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我们需要对它们应用一个 SoftMax 层。
we need to apply a SoftMax layer to them.

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正如我们所见，这会将它们转换为正数
As we can see, this transforms them into positive numbers

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总和为 1。
that sum up to 1.

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最后一步是知道哪些对应
The last step is to know which of those corresponds

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正面或负面的标签。
to the positive or the negative label.

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这是由模型配置的 id2label 字段给出的。
This is given by the id2label field of the model config.

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第一概率，索引 0，
The first probabilities, index 0,

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对应负标签，
correspond to the negative label,

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秒，索引 1，对应于正标签。
and the seconds, index 1, correspond to the positive label.

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这就是我们的分类器的构建方式
This is how our classifier built

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使用管道功能选择了那些标签
with the pipeline function picked those labels

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并计算出这些分数。
and computed those scores.

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既然你知道每个步骤是如何工作的，
Now that you know how each steps works,

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你可以轻松地根据需要调整它们。
you can easily tweak them to your needs.

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（微妙的爆炸）
(subtle blast)

