Layer Normalization, Deriving the Gradient for the Backward Pass

This post explains how to calculate the gradients of layer normalisation used for backpropagation using tensor calculus and index notation. It is part of a series on differentiating and calculating gradients in deep learning. This example is quite long and involved but combines the different concepts presented in the article series. If you have not done so, be sure to become familiar with the previous examples first.

PyTorch defines the layer normalization operation for an input matrix $X$, with shape batch size $(B)$ by hidden size $(H)$, as:

\[y=\frac{x-\mathrm{E}[x]}{\sqrt{\operatorname{Var}[x]+\epsilon}} * \gamma+\beta\]

Where the mean $\mathrm{E}[x]$ and variance $\operatorname{Var}[x]$ are calculated for each sample in a batch, and $\gamma$ and $\beta$ are learnable vector weights with lengths equal to the hidden size. $\epsilon$ is a constant usually equal to $1 \mathrm{e}-05$.

As shown previously, we can represent this using index notation:

\[\begin{aligned} m_{b} & =\frac{1}{H} \mathbf{1}_{h} x_{b h} \\ v_{b} & =\frac{1}{H} \mathbf{1}_{h}\left(x_{b h}-\mathbf{1}_{h} m_{b}\right)^{2} \\ y_{b h} & =\frac{x_{b h}-\mathbf{1}_{h} m_{b}}{\sqrt{v_{b}+\epsilon}} \gamma_{h}+\mathbf{1}_{b} \beta_{h} \end{aligned}\]

To make the problem more manageable, we are going to define additional intermediate tensor functions $\mu_{b h}$ and $\sigma_{b}$:

\[\begin{aligned} m_{b} & =\frac{1}{H} \mathbf{1}_{h} x_{b h} \\ \mu_{b h} & =x_{b h}-\mathbf{1}_{h} m_{b} \\ v_{b} & =\frac{1}{H} \mathbf{1}_{h} \mu_{b h}^{2} \\ \sigma_{b} & =\sqrt{v_{b}+\epsilon} \\ y_{b h} & =\frac{\mu_{b h}}{\sigma_{b}} \gamma_{h}+\mathbf{1}_{b} \beta_{h} \end{aligned}\]

The tensor functions above have the following dependency graph:

Gradient of Weights

Let’s start with the easier gradients $\gamma$ and $\beta$:

\[\begin{aligned} & \frac{\partial y_{b h}}{\partial \gamma_{q}}=\frac{\mu_{b h}}{\sigma_{b}} \delta_{h q} \\ & \frac{\partial y_{b h}}{\partial \beta_{q}}=\mathbf{1}_{b} \delta_{h q} \end{aligned}\]

Secondly, we find the backpropagated gradient:

\[\begin{aligned} \frac{\partial l}{\partial \gamma_{q}} & =\frac{\partial l}{\partial y_{b h}} \frac{\partial y_{b h}}{\partial \gamma_{q}}=\frac{\partial l}{\partial y_{b q}} \frac{\mu_{b q}}{\sigma_{b}} \\ \frac{\partial l}{\partial \beta_{q}} & =\frac{\partial l}{\partial y_{b h}} \frac{\partial y_{b h}}{\partial \beta_{q}}=\frac{\partial l}{\partial y_{b q}} \mathbf{1}_{b} \end{aligned}\]

Gradient of Input $X$

Directly calculating the derivative of $y_{b h}$ with respect to $x_{p q}$ is quite complex and is an order-4 tensor. However, we don’t need to construct this tensor fully since we can backpropagate the loss after each intermediate tensor function, simplifying the process. The backpropagated gradient is simpler because the loss is a scalar, meaning the gradient is, at most, an order-2 tensor.

To accomplish this, we’ll start at the end of the dependency graph and calculate the Jacobian tensor at each intermediate stage, followed by calculating the backpropagated gradient. The goal is to obtain an expression of $\partial l / \partial x_{p q}$ in terms of $\partial l / \partial y_{p q}$.

Gradient of $\sigma$

The derivative of $y_{b h}$ with respect to $\sigma_{p}$:

\[\begin{aligned} \frac{\partial y_{b h}}{\partial \sigma_{p}} & =\frac{\partial}{\partial \sigma_{p}}\left(\mu_{b h} \sigma_{b}^{-1} \gamma_{h}+\mathbf{1}_{b} \beta_{h}\right) \\ & =-\mu_{b h} \sigma_{b}^{-2} \gamma_{h} \delta_{b p} \end{aligned}\]

And the backpropagated gradient:

\[\begin{aligned} \frac{\partial l}{\partial \sigma_{p}} & =\frac{\partial l}{\partial y_{b h}} \frac{\partial y_{b h}}{\partial \sigma_{p}} \\ & =\frac{\partial l}{\partial y_{b h}}\left(-\mu_{b h} \sigma_{b}^{-2} \gamma_{h} \delta_{b p}\right) \\ & =-\frac{\partial l}{\partial y_{p h}} \mu_{p h} \sigma_{p}^{-2} \gamma_{h} \end{aligned}\]

Gradient of $v$

The derivative of $\sigma_{b}$ with respect to $v_{p}$:

\[\begin{aligned} \frac{\partial \sigma_{b}}{\partial v_{p}} & =\frac{\partial}{\partial v_{p}}\left[\left(v_{b}+\epsilon\right)^{0.5}\right] \\ & =\frac{1}{2}\left(v_{b}+\epsilon\right)^{-0.5} \delta_{b p} \\ & =\frac{\delta_{b p}}{2 \sigma_{b}} \end{aligned}\]

The backpropagated gradient:

\[\begin{aligned} \frac{\partial l}{\partial v_{p}} & =\frac{\partial l}{\partial \sigma_{b}} \frac{\partial \sigma_{b}}{\partial v_{p}} \\ & =\frac{\partial l}{\partial \sigma_{b}} \frac{\delta_{b p}}{2 \sigma_{b}} \\ & =\frac{\partial l}{\partial \sigma_{p}} \frac{1}{2 \sigma_{p}} \end{aligned}\]

Substituting in $\partial l / \partial \sigma_{p}$ from the previous step:

\[\begin{aligned} \frac{\partial l}{\partial v_{p}} & =\frac{\partial l}{\partial \sigma_{p}} \frac{1}{2 \sigma_{p}} \\ & =\left(-\frac{\partial l}{\partial y_{p h}} \mu_{p h} \sigma_{p}^{-2} \gamma_{h}\right) \frac{1}{2 \sigma_{p}} \\ & =-\frac{\partial l}{\partial y_{p h}} \frac{\mu_{p h} \gamma_{h}}{2 \sigma_{p}^{3}} \end{aligned}\]

Gradient of $\mu$

The function $\mu_{b h}$ is consumed by two functions, $v_{b}$ and $y_{b h}$, therefore we need to differentiate both functions by $\mu_{b h}$. First, the derivative of $v_{b}$ with respect to $\mu_{p q}$:

\[\begin{aligned} \frac{\partial v_{b}}{\partial \mu_{p q}} & =\frac{\partial}{\partial \mu_{p q}}\left(\frac{1}{H} \mathbf{1}_{h} \mu_{b h}^{2}\right) \\ & =\frac{2}{H} \mathbf{1}_{h} \mu_{b h} \delta_{b p} \delta_{h q} \\ & =\frac{2}{H} \mu_{b q} \delta_{b p} \end{aligned}\]

Then derivative of $y_{b h}$ with respect to $\mu_{p q}$:

\[\begin{aligned} \frac{\partial y_{b h}}{\partial \mu_{p q}} & =\frac{\partial}{\partial \mu_{p q}}\left(\frac{\mu_{b h}}{\sigma_{b}} \gamma_{h}+\mathbf{1}_{b} \beta_{h}\right) \\ & =\frac{\gamma_{h}}{\sigma_{b}} \delta_{b p} \delta_{h q} \end{aligned}\]

When applying the chain rule to obtain the backpropagated gradient, we need to include contributions from both functions:

\[\begin{aligned} \frac{\partial l}{\partial \mu_{p q}} & =\frac{\partial l}{\partial v_{b}} \frac{\partial v_{b}}{\partial \mu_{p q}}+\frac{\partial l}{\partial y_{b h}} \frac{\partial y_{b h}}{\partial \mu_{p q}} \\ & =\frac{\partial l}{\partial v_{b}}\left(\frac{2}{H} \mu_{b q} \delta_{b p}\right)+\frac{\partial l}{\partial y_{b h}}\left(\frac{\gamma_{h}}{\sigma_{b}} \delta_{b p} \delta_{h q}\right) \\ & =\frac{\partial l}{\partial v_{p}} \frac{2 \mu_{p q}}{H}+\frac{\partial l}{\partial y_{p q}} \frac{\gamma_{q}}{\sigma_{p}} \end{aligned}\]

Gradient of $m$

The derivative with respect to $m_{p}$:

\[\begin{aligned} \frac{\partial \mu_{b h}}{\partial m_{p}} & =\frac{\partial}{\partial m_{p}}\left(x_{b h}-\mathbf{1}_{h} m_{b}\right) \\ & =-\mathbf{1}_{h} \delta_{b p} \end{aligned}\]

And the backpropagated gradient:

\[\begin{aligned} \frac{\partial l}{\partial m_{p}} & =\frac{\partial l}{\partial \mu_{b h}} \frac{\partial \mu_{b h}}{\partial m_{p}} \\ & =\frac{\partial l}{\partial \mu_{b h}}\left(-\mathbf{1}_{h} \delta_{b p}\right) \\ & =-\frac{\partial l}{\partial \mu_{p h}} \mathbf{1}_{h} \end{aligned}\]

And substituting $\partial l / \partial \mu_{p h}$ derived from the previous step:

\[\begin{aligned} \frac{\partial l}{\partial m_{p}} & =-\mathbf{1}_{h}\left(\frac{\partial l}{\partial v_{p}} \frac{2 \mu_{p h}}{H}+\frac{\partial l}{\partial y_{p h}} \frac{\gamma_{h}}{\sigma_{p}}\right) \\ & =-\frac{\partial l}{\partial v_{p}} \frac{2}{H}\left(\mathbf{1}_{h} \mu_{p h}\right)-\frac{\partial l}{\partial y_{p h}} \frac{\gamma_{h}}{\sigma_{p}} \end{aligned}\]

In the first term, we have the sum \(\mathbf{1}_{h} \mu_{p h}\) which can be shown to equal zero:

\[\begin{aligned} \mathbf{1}_{h} \mu_{p h} & =\mathbf{1}_{h}\left(x_{p h}-\mathbf{1}_{h} m_{p}\right) \\ & =\mathbf{1}_{h} x_{p h}-\mathbf{1}_{h} \mathbf{1}_{h} m_{p} \\ & =H m_{p}-H m_{p} \\ & =0 \end{aligned}\]

And so, we can simplify the above expression:

\[\frac{\partial l}{\partial m_{p}}=-\frac{\partial l}{\partial y_{p h}} \frac{\gamma_{h}}{\sigma_{p}}\]

Gradient of $x$

And finally, we move onto $x_{b h}$. Two functions, $m_{b}$ and $\mu_{b h}$, consume $x_{b h}$ and so we need to consider both. First, the derivative of $m_{b}$ with respect to $x_{p q}$:

\[\begin{aligned} \frac{\partial m_{b}}{\partial x_{p q}} & =\frac{\partial}{\partial x_{p q}}\left(\frac{1}{H} \mathbf{1}_{h} x_{b h}\right) \\ & =\frac{\mathbf{1}_{q}}{H} \delta_{b p} \end{aligned}\]

And the derivative of $\mu_{b h}$ with respect to $x_{p q}$:

\[\begin{aligned} \frac{\partial \mu_{b h}}{\partial x_{p q}} & =\frac{\partial}{\partial x_{p q}}\left(x_{b h}-\mathbf{1}_{h} m_{b}\right) \\ & =\delta_{b p} \delta_{h q} \end{aligned}\]

Finally, we use the chain rule to obtain the backpropagated gradient and combine the contribution from both functions:

\[\begin{aligned} \frac{\partial l}{\partial x_{p q}} & =\frac{\partial l}{\partial m_{b}} \frac{\partial m_{b}}{\partial x_{p q}}+\frac{\partial l}{\partial \mu_{b h}} \frac{\partial \mu_{b h}}{\partial x_{p q}} \\ & =\left(-\frac{\partial l}{\partial y_{b h}} \frac{\gamma_{h}}{\sigma_{b}}\right)\left(\frac{\mathbf{1}_{q}}{H} \delta_{b p}\right)+\left(\frac{\partial l}{\partial v_{b}} \frac{2 \mu_{b h}}{H}+\frac{\partial l}{\partial y_{b h}} \frac{\gamma_{h}}{\sigma_{b}}\right)\left(\delta_{b p} \delta_{h q}\right) \\ & =-\frac{\mathbf{1}_{q}}{H} \frac{\partial l}{\partial y_{p h}} \frac{\gamma_{h}}{\sigma_{p}}+\frac{2}{H} \frac{\partial l}{\partial v_{p}} \mu_{p q}+\frac{\partial l}{\partial y_{p q}} \frac{\gamma_{q}}{\sigma_{p}} \end{aligned}\]

The goal is to obtain an expression of $\partial l / \partial x_{p q}$ in terms of $\partial l / \partial y_{p q}$, and so we substituent $\partial l / \partial v_{p}$ using the previously derived expression and rearranging the terms to obtain the final result:

\[\begin{aligned} \frac{\partial l}{\partial x_{p q}} & =-\frac{\mathbf{1}_{q}}{H} \frac{\partial l}{\partial y_{p h}} \frac{\gamma_{h}}{\sigma_{p}}+\frac{2}{H}\left(-\frac{\partial l}{\partial y_{p h}} \frac{\mu_{p h} \gamma_{h}}{2 \sigma_{p}^{3}}\right) \mu_{p q}+\frac{\partial l}{\partial y_{p q}} \frac{\gamma_{q}}{\sigma_{p}} \\ & =\frac{\partial l}{\partial y_{p q}} \frac{\gamma_{q}}{\sigma_{p}}-\frac{\partial l}{\partial y_{p h}} \frac{\gamma_{h}}{H}\left(\frac{\mathbf{1}_{q}}{\sigma_{p}}+\frac{\mu_{p h} \mu_{p q}}{\sigma_{p}^{3}}\right) \end{aligned}\]

Conclusion

Bringing the results together:

\[\begin{gathered} \frac{\partial l}{\partial \gamma_{q}}=\frac{\partial l}{\partial y_{b q}} \frac{\mu_{b q}}{\sigma_{b}} \\ \frac{\partial l}{\partial \beta_{q}}=\frac{\partial l}{\partial y_{b q}} \mathbf{1}_{b} \\ \frac{\partial l}{\partial x_{p q}}=\frac{\partial l}{\partial y_{p q}} \frac{\gamma_{q}}{\sigma_{p}}-\frac{\partial l}{\partial y_{p h}} \frac{\gamma_{h}}{H}\left(\frac{\mathbf{1}_{q}}{\sigma_{p}}+\frac{\mu_{p h} \mu_{p q}}{\sigma_{p}^{3}}\right) \end{gathered}\]

Next

Further examples of calculating gradients using tensor calculus and index notation can be found on the intro page.