subtitles/en/68_data-collators-a-tour.srt

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(sticker popping)

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In a lot of our examples,

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you're going to see DataCollators
popping up over and over.

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They're used in both PyTorch
and TensorFlow workflows,

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and maybe even in JAX,

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but no-one really knows
what's happening in JAX.

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We do have a research
team working on it though,

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so maybe they'll tell us soon.

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But coming back on topic.

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What are data collators?

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Data collators collate data.

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That's not that helpful.

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But to be more specific, they
put together a list of samples

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into a single training minibatch.

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For some tasks,

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the data collator can
be very straightforward.

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For example, when you're
doing sequence classification,

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all you really need
from your data collator

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is that it pads your
samples to the same length

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and concatenates them
into a single Tensor.

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But for other workflows, data
collators can be quite complex

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as they handle some of the preprocessing

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needed for that particular task.

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So, if you want to use a data collator,

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for PyTorch users, you
usually pass the data collator

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to your Trainer object.

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In TensorFlow, it's a bit different.

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The easiest way to use a data collator

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is to pass it to the to_tf_dataset
method of your dataset.

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And this will give you a
tensorflow_tf_data.dataset

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that you can then pass to model.fit.

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You'll see these approaches
used in the examples

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and notebooks throughout this course.

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Also note that all of our collators

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take a return_tensors argument.

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You can set this to "pt"
to get PyTorch Tensors,

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"tf" to get TensorFlow Tensors,

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or "np" to get Numpy arrays.

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For backward compatibility reasons,

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the default value is "pt",

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so PyTorch users don't even
have to set this argument

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most of the time.

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And so as a result, they're
often totally unaware

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that this argument even exists.

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We can learn something from this

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which is that the
beneficiaries of privilege

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are often the most blind to its existence.

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But okay, coming back.

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Let's see how some specific
data collators work in action.

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Although again, remember if none

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of the built-in data
collators do what you need,

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you can always write your own
and they're often quite short.

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So first, we'll see the
"basic" data collators.

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These are DefaultDataCollator
and DataCollatorWithPadding.

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These are the ones you should use

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if your labels are straightforward

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and your data doesn't need
any special processing

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before being ready for training.

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Notice that because different models

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have different padding tokens,

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DataCollatorWithPadding will
need your model's Tokenizer

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so it knows how to pad sequences properly.

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The default data collator
doesn't need a Tokenizer to work,

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but it will as a result throw an error

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unless all of your sequences
are the same length.

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So, you should be aware of that.

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Moving on though.

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A lot of the other data collators

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aside from the basic two are,

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they're usually designed to
handle one specific task.

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And so, I'm going to show a couple here.

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These are
DataCollatorForTokenClassification

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and DataCollatorForSeqToSeq.

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And the reason these tasks
need special collators

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is because their labels
are variable in length.

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In token classification there's
one label for each token,

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and so the length of the labels

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is the length of the sequence.

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While in SeqToSeq the labels
are a sequence of tokens

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that can be variable length,

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that can be very different

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from the length of the input sequence.

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So in both of these cases, we
handle collating that batch

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by padding the labels as well,

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as you can see here in this example.

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So, inputs and the labels
will need to be padded

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if we want to join
samples of variable length

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into the same minibatch.

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That's exactly what the data collators

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and that's exactly what these
data collators will do for us

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you know, for this particular task.

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So, there's one final data collator

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I want to show you as
well just in this lecture.

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And that's the
DataCollatorForLanguageModeling.

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So, it's very important, and it's firstly,

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because language models
are just so foundational

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to do for everything we
do with NLP these days.

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But secondly, because it has two modes

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that do two very different things.

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So you choose which mode you
want with the mlm argument.

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Set it to True for
masked language modeling,

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and set it to False for
causal language modeling.

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So, collating data for
causal language modeling

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is actually quite straightforward.

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The model is just making predictions

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for what token comes
next, and so your labels

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are more or less just
a copy of your inputs,

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and the collator will handle that

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and ensure that the inputs and
labels are padded correctly.

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When you set mlm to True though,

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you get quite different behavior,

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that's different from
any other data collator,

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and that's because setting mlm to True

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means masked language modeling

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and that means the labels need to be,

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you know, the inputs need to be masked.

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So, what does that look like?

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So, recall that in
masked language modeling,

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the model is not predicting the next word,

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instead we randomly mask out some tokens

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and the model predicts
all of them at once.

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So, it tries to kinda fill in the blanks

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for those masked tokens.

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But the process of random
masking is surprisingly complex.

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If we follow the protocol
from the original BERT paper,

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we need to replace some
tokens with a masked token,

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some other tokens with a random token,

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and then keep a third
set of tokens unchanged.

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Yeah, this is not the lecture

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to go into the specifics
of that or why we do it.

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You can always check out
the original BERT paper

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if you're curious.

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It's well written. It's
easy to understand.

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The main thing to know here
is that it can be a real pain

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and quite complex to
implement that yourself.

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But DataCollatorForLanguageModeling
will do it for you

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when you set mlm to True.

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And that's an example
of the more intricate

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preprocessing that some
of our data collators do.

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And that's it!

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So, this covers the most
commonly used data collators

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and the tasks they're used for.

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And hopefully, now you'll know
when to use data collators

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and which one to choose
for your specific task.

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(whooshing sound)