subtitles/en/66_the-post-processing-step-in-question-answering-(pytorch).srt

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- The post-processing step
in a question answering task.

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When doing question answering,

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the processing of the initial dataset

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implies splitting examples
in several features,

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which may or may not contain the answer.

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Passing those features through the model

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will give us logits for the
start and end positions,

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since our labels are
the indices of the token

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that correspond to the
start and end the answer.

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We must then somehow convert
those logits into an answer,

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and then pick one of the various
answers each feature gives

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to be the answer for a given example.

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For the processing step,

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you should refer to
the video linked below.

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It's not very different for validation,

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we just need to add a few lines
to keep track of two things.

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Instead of discarding the
offset mappings, we keep them,

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and also include in them

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the information of where the context is

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by setting the offsets
of the special tokens

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and the question to None.

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Then we also keep track of the
example ID for each feature,

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to be able to map back feature

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to the examples that they originated from.

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If you don't want to
compute the validation loss,

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you won't need to include
all the special code

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that we used to create the labels.

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With this done, we can apply
that preprocessing function

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using the map method.

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We take the SQUAD dataset

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like in the preprocessing
for question-answering video.

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Once this is done, the next
step is to create our model.

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We use the default model

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behind the question-answering
pipeline here,

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but you should use any
model you want to evaluate.

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We'll run a manual evaluation loop,

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so we create a PyTorch
DataLoader with our features.

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With it, we can compute and gather

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all the start and end logits like this,

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with a standard PyTorch evaluation loop.

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With this done, we can really
dive into the post-processing.

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First, we'll need a map
from example to features,

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which we can create like this.

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Now, for the main part
of the post-processing,

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let's see how to extract
an answer from the logits.

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We could just take the best index

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for the start and end logits and be done,

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but if our model predicts
something impossible,

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like tokens in the question,

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we'll look at more of the logits.

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Note that in the
question-answering pipeline,

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we attributed score to each answer

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based on the probabilities,

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which we did not compute here.

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In terms of logits, the
multiplication we had in the scores

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becomes an addition.

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To go fast,

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we don't look at all possible
start and end logits,

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but the 20 best one is enough.

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We ignore the logits that
spawn impossible answers

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or answer that are too long.

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As we saw in the
preprocessing, the labels 0,0

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correspond to a no answer.

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Otherwise we use the offsets

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to get the answer inside the context.

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Let's have a look at the predicted answer

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for the first feature,

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which is the answer with the best score

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or the best logit score

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since the SoftMax is
an increasing function.

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The model got it right.

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Next we just have to loop
this for every example,

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picking for each the answer
with the best logit score

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in all the features the example generated.

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Now you know how to get answers
from your model prediction.

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