groupNormalizationLayer

Group normalization layer

Description

A group normalization layer normalizes a mini-batch of data across grouped subsets of channels for each observation independently. To speed up training of the convolutional neural network and reduce the sensitivity to network initialization, use group normalization layers between convolutional layers and nonlinearities, such as ReLU layers.

After normalization, the layer scales the input with a learnable scale factor γ and shifts by a learnable offset β.

Creation

Syntax

``layer = groupNormalizationLayer(numGroups)``
``layer = groupNormalizationLayer(numGroups,Name,Value)``

Description

example

````layer = groupNormalizationLayer(numGroups)` creates a group normalization layer.```

example

````layer = groupNormalizationLayer(numGroups,Name,Value)` creates a group normalization layer and sets the optional `'Epsilon'`, Parameters and Initialization, Learn Rate and Regularization, and `Name` properties using one or more name-value pair arguments. You can specify multiple name-value pair arguments. Enclose each property name in quotes.```

Input Arguments

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Number of groups into which to divide the channels of the input data, specified as one of the following:

• Positive integer – Divide the incoming channels in to the specified number of groups. The specified number of groups must divide the number of channels of the input data exactly.

• `'all-channels'` – Group all incoming channels into a single group. This is also known as layer normalization. Alternatively, use `layerNormalizationLayer`.

• `'channel-wise'` – Treat all incoming channels as separate groups. This is also known as instance normalization. Alternatively, use `instanceNormalizationLayer`.

Properties

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Group Normalization

Constant to add to the mini-batch variances, specified as a numeric scalar equal to or larger than `1e-5`.

The layer adds this constant to the mini-batch variances before normalization to ensure numerical stability and avoid division by zero.

Number of input channels, specified as `'auto'` or a positive integer.

This property is always equal to the number of channels of the input to the layer. If `NumChannels` equals `'auto'`, then the software automatically determines the correct value for the number of channels at training time.

Parameters and Initialization

Function to initialize the channel scale factors, specified as one of the following:

• `'ones'` – Initialize the channel scale factors with ones.

• `'zeros'` – Initialize the channel scale factors with zeros.

• `'narrow-normal'` – Initialize the channel scale factors by independently sampling from a normal distribution with zero mean and standard deviation 0.01.

• Function handle – Initialize the channel scale factors with a custom function. If you specify a function handle, then the function must be of the form `scale = func(sz)`, where `sz` is the size of the scale. For an example, see Specify Custom Weight Initialization Function.

The layer only initializes the channel scale factors when the `Scale` property is empty.

Data Types: `char` | `string` | `function_handle`

Function to initialize the channel offsets, specified as one of the following:

• `'zeros'` – Initialize the channel offsets with zeros.

• `'ones'` – Initialize the channel offsets with ones.

• `'narrow-normal'` – Initialize the channel offsets by independently sampling from a normal distribution with zero mean and standard deviation 0.01.

• Function handle – Initialize the channel offsets with a custom function. If you specify a function handle, then the function must be of the form `offset = func(sz)`, where `sz` is the size of the scale. For an example, see Specify Custom Weight Initialization Function.

The layer only initializes the channel offsets when the `Offset` property is empty.

Data Types: `char` | `string` | `function_handle`

Channel scale factors γ, specified as a numeric array.

The channel scale factors are learnable parameters. When training a network, if `Scale` is nonempty, then `trainNetwork` uses the `Scale` property as the initial value. If `Scale` is empty, then `trainNetwork` uses the initializer specified by `ScaleInitializer`.

At training time, `Scale` is one of the following:

• For 2-D image input, a numeric array of size 1-by-1-by-`NumChannels`

• For 3-D image input, a numeric array of size 1-by-1-by-1-by-`NumChannels`

• For feature or sequence input, a numeric array of size `NumChannels`-by-1

Channel offsets β, specified as a numeric array.

The channel offsets are learnable parameters. When training a network, if `Offset` is nonempty, then `trainNetwork` uses the `Offset` property as the initial value. If `Offset` is empty, then `trainNetwork` uses the initializer specified by `OffsetInitializer`.

At training time, `Offset` is one of the following:

• For 2-D image input, a numeric array of size 1-by-1-by-`NumChannels`

• For 3-D image input, a numeric array of size 1-by-1-by-1-by-`NumChannels`

• For feature or sequence input, a numeric array of size `NumChannels`-by-1

Learn Rate and Regularization

Learning rate factor for the scale factors, specified as a nonnegative scalar.

The software multiplies this factor by the global learning rate to determine the learning rate for the scale factors in a layer. For example, if `ScaleLearnRateFactor` is `2`, then the learning rate for the scale factors in the layer is twice the current global learning rate. The software determines the global learning rate based on the settings specified with the `trainingOptions` function.

Learning rate factor for the offsets, specified as a nonnegative scalar.

The software multiplies this factor by the global learning rate to determine the learning rate for the offsets in a layer. For example, if `OffsetLearnRateFactor` equals `2`, then the learning rate for the offsets in the layer is twice the current global learning rate. The software determines the global learning rate based on the settings specified with the `trainingOptions` function.

L2 regularization factor for the scale factors, specified as a nonnegative scalar.

The software multiplies this factor by the global L2 regularization factor to determine the learning rate for the scale factors in a layer. For example, if `ScaleL2Factor` is 2, then the L2 regularization for the offsets in the layer is twice the global L2 regularization factor. You can specify the global L2 regularization factor using the `trainingOptions` function.

L2 regularization factor for the offsets, specified as a nonnegative scalar.

The software multiplies this factor by the global L2 regularization factor to determine the learning rate for the offsets in a layer. For example, if `OffsetL2Factor` is 2, then the L2 regularization for the offsets in the layer is twice the global L2 regularization factor. You can specify the global L2 regularization factor using the `trainingOptions` function.

Layer

Layer name, specified as a character vector or a string scalar. To include a layer in a layer graph, you must specify a nonempty, unique layer name. If you train a series network with the layer and `Name` is set to `''`, then the software automatically assigns a name to the layer at training time.

Data Types: `char` | `string`

Number of inputs of the layer. This layer accepts a single input only.

Data Types: `double`

Input names of the layer. This layer accepts a single input only.

Data Types: `cell`

Number of outputs of the layer. This layer has a single output only.

Data Types: `double`

Output names of the layer. This layer has a single output only.

Data Types: `cell`

Examples

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Create a group normalization layer that normalizes incoming data across three groups of channels. Name the layer `'groupnorm'`.

`layer = groupNormalizationLayer(3,'Name','groupnorm')`
```layer = GroupNormalizationLayer with properties: Name: 'groupnorm' NumChannels: 'auto' Hyperparameters NumGroups: 3 Epsilon: 1.0000e-05 Learnable Parameters Offset: [] Scale: [] Show all properties ```

Include a group normalization layer in a `Layer` array. Normalize the incoming 20 channels in four groups.

```layers = [ imageInputLayer([28 28 3]) convolution2dLayer(5,20) groupNormalizationLayer(4) reluLayer maxPooling2dLayer(2,'Stride',2) fullyConnectedLayer(10) softmaxLayer classificationLayer]```
```layers = 8x1 Layer array with layers: 1 '' Image Input 28x28x3 images with 'zerocenter' normalization 2 '' Convolution 20 5x5 convolutions with stride [1 1] and padding [0 0 0 0] 3 '' Group Normalization Group normalization 4 '' ReLU ReLU 5 '' Max Pooling 2x2 max pooling with stride [2 2] and padding [0 0 0 0] 6 '' Fully Connected 10 fully connected layer 7 '' Softmax softmax 8 '' Classification Output crossentropyex ```

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Algorithms

The group normalization operation normalizes the elements xi of the input by first calculating the mean μG and variance σG2 over spatial, time, and grouped subsets of the channel dimensions for each observation independently. Then, it calculates the normalized activations as

`${\stackrel{^}{x}}_{i}=\frac{{x}_{i}-{\mu }_{G}}{\sqrt{{\sigma }_{G}^{2}+\epsilon }},$`

where ϵ is a constant that improves numerical stability when the variance is very small. To allow for the possibility that inputs with zero mean and unit variance are not optimal for the operations that follow group normalization, the group normalization operation further shifts and scales the activations using the transformation

`${y}_{i}=\gamma {\stackrel{^}{x}}_{i}+\beta ,$`

where the offset β and scale factor γ are learnable parameters that are updated during network training.

References

[1] Wu, Yuxin, and Kaiming He. “Group Normalization.” ArXiv:1803.08494 [Cs], June 11, 2018. http://arxiv.org/abs/1803.08494.

Introduced in R2020b