View source on GitHub |
Layer normalization layer (Ba et al., 2016).
Inherits From: Layer
, Operation
tf.keras.layers.LayerNormalization(
axis=-1,
epsilon=0.001,
center=True,
scale=True,
rms_scaling=False,
beta_initializer='zeros',
gamma_initializer='ones',
beta_regularizer=None,
gamma_regularizer=None,
beta_constraint=None,
gamma_constraint=None,
**kwargs
)
Used in the notebooks
Used in the tutorials |
---|
Normalize the activations of the previous layer for each given example in a batch independently, rather than across a batch like Batch Normalization. i.e. applies a transformation that maintains the mean activation within each example close to 0 and the activation standard deviation close to 1.
If scale
or center
are enabled, the layer will scale the normalized
outputs by broadcasting them with a trainable variable gamma
, and center
the outputs by broadcasting with a trainable variable beta
. gamma
will
default to a ones tensor and beta
will default to a zeros tensor, so that
centering and scaling are no-ops before training has begun.
So, with scaling and centering enabled the normalization equations are as follows:
Let the intermediate activations for a mini-batch to be the inputs
.
For each sample x_i
in inputs
with k
features, we compute the mean and
variance of the sample:
mean_i = sum(x_i[j] for j in range(k)) / k
var_i = sum((x_i[j] - mean_i) ** 2 for j in range(k)) / k
and then compute a normalized x_i_normalized
, including a small factor
epsilon
for numerical stability.
x_i_normalized = (x_i - mean_i) / sqrt(var_i + epsilon)
And finally x_i_normalized
is linearly transformed by gamma
and beta
,
which are learned parameters:
output_i = x_i_normalized * gamma + beta
gamma
and beta
will span the axes of inputs
specified in axis
, and
this part of the inputs' shape must be fully defined.
For example:
layer = keras.layers.LayerNormalization(axis=[1, 2, 3])
layer.build([5, 20, 30, 40])
print(layer.beta.shape)
(20, 30, 40)
print(layer.gamma.shape)
(20, 30, 40)
Note that other implementations of layer normalization may choose to define
gamma
and beta
over a separate set of axes from the axes being
normalized across. For example, Group Normalization
(Wu et al. 2018) with group size of 1
corresponds to a Layer Normalization that normalizes across height, width,
and channel and has gamma
and beta
span only the channel dimension.
So, this Layer Normalization implementation will not match a Group
Normalization layer with group size set to 1.
Args | |
---|---|
axis
|
Integer or List/Tuple. The axis or axes to normalize across.
Typically, this is the features axis/axes. The left-out axes are
typically the batch axis/axes. -1 is the last dimension in the
input. Defaults to -1 .
|
epsilon
|
Small float added to variance to avoid dividing by zero. Defaults to 1e-3. |
center
|
If True, add offset of beta to normalized tensor. If False,
beta is ignored. Defaults to True .
|
scale
|
If True, multiply by gamma . If False, gamma is not used.
When the next layer is linear (also e.g. nn.relu ), this can be
disabled since the scaling will be done by the next layer.
Defaults to True .
|
rms_scaling
|
If True, center and scale are ignored, and the
inputs are scaled by gamma and the inverse square root
of the square of all inputs. This is an approximate and faster
approach that avoids ever computing the mean of the input.
|
beta_initializer
|
Initializer for the beta weight. Defaults to zeros. |
gamma_initializer
|
Initializer for the gamma weight. Defaults to ones. |
beta_regularizer
|
Optional regularizer for the beta weight. None by default. |
gamma_regularizer
|
Optional regularizer for the gamma weight. None by default. |
beta_constraint
|
Optional constraint for the beta weight. None by default. |
gamma_constraint
|
Optional constraint for the gamma weight. None by default. |
**kwargs
|
Base layer keyword arguments (e.g. name and dtype ).
|
Reference:
Methods
from_config
@classmethod
from_config( config )
Creates a layer from its config.
This method is the reverse of get_config
,
capable of instantiating the same layer from the config
dictionary. It does not handle layer connectivity
(handled by Network), nor weights (handled by set_weights
).
Args | |
---|---|
config
|
A Python dictionary, typically the output of get_config. |
Returns | |
---|---|
A layer instance. |
symbolic_call
symbolic_call(
*args, **kwargs
)