HuggingFace.Quantization_in_Depth/Transcript/2_Quantize and De-quantize a Tensor_HFQD.txt

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In this lesson, you will dive deep
into the theory of linear quantization.

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You will implement from scratch the
asymmetric variant of linear quantization.

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You will also learn about
the scaling factor and the zero point.

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Let's get started.

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Quantization refers to the process

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of mapping a large
set to a smaller set of values.

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There are many quantization techniques.

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In this course,
we will focus only on linear quantization.

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Let's have a look at an example.

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On your left you can see
the original tensor in floating .32.

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And we have the quantized tensor
on the right.

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The quantized tensor is quantized
in torch.int8,

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and we use linear quantization
to get this tensor.

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We will see in this lesson
how we get this quantized tensor.

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But also
how do we get back to the original tensor.

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Let's have a quick recap on what
we can quantize in a neural network.

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In a neural network
you can quantize the weights.

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That is to say,
the neural network parameters.

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But you can also
quantize the activations.

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The activations are

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values that propagates
through the layers of the neural network.

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And if you quantize a neural network
after it has been trained,

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you are doing something called
post-training quantization.

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There are multiple advantages
of quantization.

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Of course you get a smaller model,
but you can also get speed gains

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from the memory bandwidth
and faster operation, such as the matrix

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to matrix multiplication
and the matrix to vector multiplication.

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We will see why it is
the case in the next lesson

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when we talked about how to perform
inference with a quantized model.

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There are many challenges to quantization.

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We will deep dive into these challenges
in the last lesson of this short course.

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But now I'm going to give you a quick
preview of these challenges.

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Now, let's jump on the theory of linear
quantization.

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Linear quantization uses a linear
mapping to map the higher precision range.

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For example, floating point 32 to a lower
precision range for example int8.

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There are two parameters
in linear quantization.

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We have the scale S and the zero point z.

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The scale is stored in the same data
type as the original tensor,

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and z is stored in the same datatype
as the quantized tensor.

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We will see why in the next few slides.

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Now let's check a quick example.

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Let's say the scale is equal to two
and the zero point is equal to zero.

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If we have a quantized value of ten,
the dequantized value

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would be equal to 2(q-0),

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which will be equal to 2*10,
which will be equal to 20.

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If we look at the example

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we presented in the first few slides,
we would have something like this:

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So, here we have the original tensor.

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We have the quantized tensor here.

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And the zero point is equals to -77.

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And the scale is equal to 3.58.

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We will see how we get the zero point
and the scale

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in the next few slides.

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But first, we have the original tensor

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and we need to quantize this tensor.

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So, how do we get Q? If you remember well,

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the relationship is r=s(q-z).

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So how do we get q?
To get the quantized tensor

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we just need to isolate q
and we get the following formula.

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So, in order to get the quantized tensor,
as I said before, you need to isolate q.

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So first, we have r=s(q-z).

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We need to pass s to the left side
by dividing it by s.

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Then we put the zero point
on the other side

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by adding a z on this side
and on this side.

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So we get the following results.

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As you know the quantized tensor is on

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the specific d-type
which can be eight-bit integers.

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So we need to round that number.

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And the last step would be to cast this
value to the correct d-type such as int8.

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Let's code that. In this classroom
the libraries have

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already been installed for you.

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But if you are running this
on your own machine, all you need to do

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is to type the following
command in order to install torch.

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Pip install torch.

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Since in this classroom
the libraries have already been installed,

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I won't be running this comment,
so I will just comment it out.

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Now, all we do need to do
is to import torch.

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Now, let's code the function

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that will give us the quantize tensor

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knowing the scale and the zero points.

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So, we define a function called linear

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q for quantization with

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scale and zero point.

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This function
will take multiple arguments.

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So we have the tensor.

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We have the scale.

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We have the zero point.

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And we also need to define the d type
which will be equal

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by default to torch.int8.

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So, the first step is to get the scaled
and shifted tensor.

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As you can see in the formula right here.

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So, (r/s+z).

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So we are going to first calculate that.

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So this specific tensor

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will be equal to tensor

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divided by scale,

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plus zero points.

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We need to run the tensor.

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As you can see in the formula.

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So we will just create the variable

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around the tensor.

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Which will be equal to torch.round.

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The round method will enable

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the torch.round methods.

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We round the tensor that we pass.

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And the last step
is to make sure that our rounded tensor

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is between the minimum quantized value
and the maximum quantized value.

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And then we can finally cast it
to the specified type.

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Let's do that.

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So first,

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we need to get the minimum quantized value
and the maximum quantized value.

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So to get the minimum quantized value

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we will use the torch.iinfo methods.

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We will pass the dtype that we define

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in the attribute of the function.

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And to get the minimum
we just need to pass min.

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We do the same
thing for the maximum value.

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Now, we can define the quantized tensor

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which will be

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=rounded_tensor.clamp(q_min_max).

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And we can cast this tensor

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to the quantized dtype you want,

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such as int8.

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And the last step

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is to return the quantized tensor.

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Now that we have coded

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our function let's test or implementation.

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So we'll define the test tensor.

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We will define the same tensor
that you saw in the example on the slides.

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And we will assign random values
for scale and zero point.

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Since we don't know how to get them yet.

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So I'll just put scale equals to 3.5

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and the zero point to -70.

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Then let's get our quantized tensor
by calling the linear

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q with scale and zero
point function that we just coded.

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And we need to pass

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the test tensor,

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the scale that we and the zero point

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we defined earlier.

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And now let's check the quantized tensor.

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As you can see
we managed to quantize the tensor.

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And we can see that the dtype of
the tensor is indeed torch.int8.

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So now that we have our quantized tensor,
let's dequantize it

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to see how precise the quantization is.

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So, the quantization
formula is the one we saw in the slides.

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We have r=(q-z).

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And we will use just that.

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So, to get the dequantized tensor
we will just do

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scale * (quantized_tensor.float()
because we need to cast it to a float.

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Otherwise we will get weird behaviors
with underflow and overflows.

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Since we are doing a subtraction
between two int8 integers.

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Let's check the results.

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So we get these following values.

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But let's check
what happens if we don't cast

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quantized tensor to float.

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What we will get is the following results.

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Which is not the same

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as you can see here we have 686
and now we have -210.

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Now, let's put it into a function
called linear

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dequantization.

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So, for the linear

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dequantization function
we need to put as arguments

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the quantized tensor
the scale and the zero point.

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And then we just need to return

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what we called it above.

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As you can see on the right,

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you have the quantization error tensor.

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We have for some entries
pretty small values,

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which shows that the quantization worked
pretty well.

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But, as you can see here
we have also pretty big values.

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To get the quantization error tensor

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we just subtract the original tensor
and the dequantized tensor

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and we take the absolute
value of the entire matrix.

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And at the end, as you can see,

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we end up with a quantization
error of around 170.

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The error is quite high
because in this example

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we assign a random value to scale and zero
points.

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Let's cover in the next section
how to find out those optimal values.