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(subtle blast)

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- What happens inside
the pipeline function?

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In this video, we will look
at what actually happens

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when we use the pipeline function

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of the Transformers library.

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More specifically, we will look at

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the sentiment analysis pipeline,

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and how it went from the
two following sentences

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to the positive and negative labels

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with their respective scores.

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As we have seen in the pipeline video,

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there are three stages in the pipeline.

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First, we convert the raw texts to numbers

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the model can make sense
of, using a tokenizer.

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Then, those numbers go through the model,

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which outputs logits.

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Finally, the post-processing steps

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transforms those logits
into labels and score.

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Let's look in detail at those three steps,

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and how to replicate them
using the Transformers library,

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beginning with the first
stage, tokenization.

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The tokenization process
has several steps.

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First, the text is split into
small chunks called token.

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They can be words, parts of
words or punctuation symbols.

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Then the tokenizer will
had some special tokens

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if the model expect them.

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Here, the model used expects
a CLS token at the beginning

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and a SEP token at the end
of the sentence to classify.

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Lastly, the tokenizer matches
each token to its unique ID

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in the vocabulary of the pretrained model.

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To load such a tokenizer,

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the Transformers library
provides the AutoTokenizer API.

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The most important method of
this class is from_pretrained,

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which will download and
cache the configuration

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and the vocabulary associated
to a given checkpoint.

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Here, the checkpoint used by default

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for the sentiment analysis pipeline

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is distilbert base uncased
finetuned sst2 English,

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which is a bit of a mouthful.

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We instantiate a tokenizer
associated with that checkpoint,

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then feed it the two sentences.

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Since those two sentences
are not of the same size,

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we will need to pad the shortest one

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to be able to build an array.

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This is done by the tokenizer

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with the option padding=True.

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With truncation=True, we
ensure that any sentence longer

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than the maximum the model
can handle is truncated.

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Lastly, the return_tensors
option tells the tokenizer

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to return a PyTorch tensor.

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Looking at the result,

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we see we have a dictionary with two keys.

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Input IDs contains the
IDs of both sentences,

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with zeros where the padding is applied.

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The second key, attention mask,

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indicates where padding has been applied,

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so the model does not pay attention to it.

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This is all what is inside
the tokenization step.

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Now let's have a look at
the second step, the model.

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As for the tokenizer,
there is an AutoModel API,

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with a from_pretrained method.

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It will download and cache
the configuration of the model

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as well as the pretrained weights.

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However, the AutoModel
API will only instantiate

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the body of the model,

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that is, the part of
the model that is left

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once the pretraining head is removed.

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It will output a high-dimensional tensor

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that is a representation
of the sentences passed,

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but which is not directly useful

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for our classification problem.

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Here the tensor has two sentences,

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each of sixteen token,

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and the last dimension is the
hidden size of our model, 768.

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To get an output linked to
our classification problem,

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we need to use the
AutoModelForSequenceClassification class.

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It works exactly as the AutoModel class,

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except that it will build a model

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with a classification head.

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There is one auto class
for each common NLP task

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in the Transformers library.

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Here, after giving our
model the two sentences,

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we get a tensor of size two by two;

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one result for each sentence
and for each possible label.

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Those outputs are not probabilities yet.

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We can see they don't sum to 1.

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This is because each model
of the Transformers library

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returns logits.

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To make sense of those logits,

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we need to dig into the third

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and last step of the
pipeline, post-processing.

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To convert logits into probabilities,

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we need to apply a SoftMax layer to them.

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As we can see, this transforms
them into positive numbers

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that sum up to 1.

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The last step is to know
which of those corresponds

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to the positive or the negative label.

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This is given by the id2label
field of the model config.

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The first probabilities, index 0,

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correspond to the negative label,

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and the seconds, index 1,
correspond to the positive label.

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This is how our classifier built

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with the pipeline function
picked those labels

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and computed those scores.

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Now that you know how each steps works,

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you can easily tweak them to your needs.

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(subtle blast)