spaCy/website/docs/usage/layers-architectures.md

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Layers and Model Architectures	Power spaCy components with custom neural networks	Type Signatures type-sigs Swapping Architectures swap-architectures PyTorch & TensorFlow frameworks Custom Thinc Models thinc Trainable Components components	/usage/projects

Example

from thinc.api import Model, chain

@spacy.registry.architectures.register("model.v1")
def build_model(width: int, classes: int) -> Model:
    tok2vec = build_tok2vec(width)
    output_layer = build_output_layer(width, classes)
    model = chain(tok2vec, output_layer)
    return model

A model architecture is a function that wires up a Thinc Model instance. It describes the neural network that is run internally as part of a component in a spaCy pipeline. To define the actual architecture, you can implement your logic in Thinc directly, or you can use Thinc as a thin wrapper around frameworks such as PyTorch, TensorFlow and MXNet. Each Model can also be used as a sublayer of a larger network, allowing you to freely combine implementations from different frameworks into a single model.

spaCy's built-in components require a Model instance to be passed to them via the config system. To change the model architecture of an existing component, you just need to update the config so that it refers to a different registered function. Once the component has been created from this config, you won't be able to change it anymore. The architecture is like a recipe for the network, and you can't change the recipe once the dish has already been prepared. You have to make a new one.

### config.cfg (excerpt)
[components.tagger]
factory = "tagger"

[components.tagger.model]
@architectures = "model.v1"
width = 512
classes = 16

Type signatures

Example

from typing import List
from thinc.api import Model, chain
from thinc.types import Floats2d
def chain_model(
    tok2vec: Model[List[Doc], List[Floats2d]],
    layer1: Model[List[Floats2d], Floats2d],
    layer2: Model[Floats2d, Floats2d]
) -> Model[List[Doc], Floats2d]:
    model = chain(tok2vec, layer1, layer2)
    return model

The Thinc Model class is a generic type that can specify its input and output types. Python uses a square-bracket notation for this, so the type Model[List, Dict] says that each batch of inputs to the model will be a list, and the outputs will be a dictionary. You can be even more specific and write for instanceModel[List[Doc], Dict[str, float]] to specify that the model expects a list of Doc objects as input, and returns a dictionary mapping of strings to floats. Some of the most common types you'll see are: ​

Type	Description
List[Doc]	A batch of Doc objects. Most components expect their models to take this as input.
Floats2d	A two-dimensional numpy or cupy array of floats. Usually 32-bit.
Ints2d	A two-dimensional numpy or cupy array of integers. Common dtypes include uint64, int32 and int8.
List[Floats2d]	A list of two-dimensional arrays, generally with one array per Doc and one row per token.
Ragged	A container to handle variable-length sequence data in an unpadded contiguous array.
Padded	A container to handle variable-length sequence data in a padded contiguous array.

The model type signatures help you figure out which model architectures and components can fit together. For instance, the TextCategorizer class expects a model typed Model[List[Doc], Floats2d], because the model will predict one row of category probabilities per Doc. In contrast, the Tagger class expects a model typed Model[List[Doc], List[Floats2d]], because it needs to predict one row of probabilities per token.

There's no guarantee that two models with the same type signature can be used interchangeably. There are many other ways they could be incompatible. However, if the types don't match, they almost surely won't be compatible. This little bit of validation goes a long way, especially if you configure your editor or other tools to highlight these errors early. The config file is also validated at the beginning of training, to verify that all the types match correctly.

If you're using a modern editor like Visual Studio Code, you can set up mypy with the custom Thinc plugin and get live feedback about mismatched types as you write code.

Swapping model architectures

If no model is specified for the TextCategorizer, the TextCatEnsemble architecture is used by default. This architecture combines a simple bag-of-words model with a neural network, usually resulting in the most accurate results, but at the cost of speed. The config file for this model would look something like this:

### config.cfg (excerpt)
[components.textcat]
factory = "textcat"
labels = []

[components.textcat.model]
@architectures = "spacy.TextCatEnsemble.v1"
exclusive_classes = false
pretrained_vectors = null
width = 64
conv_depth = 2
embed_size = 2000
window_size = 1
ngram_size = 1
dropout = 0
nO = null

spaCy has two additional built-in textcat architectures, and you can easily use those by swapping out the definition of the textcat's model. For instance, to use the simple and fast bag-of-words model TextCatBOW, you can change the config to:

### config.cfg (excerpt) {highlight="6-10"}
[components.textcat]
factory = "textcat"
labels = []

[components.textcat.model]
@architectures = "spacy.TextCatBOW.v1"
exclusive_classes = false
ngram_size = 1
no_output_layer = false
nO = null

For details on all pre-defined architectures shipped with spaCy and how to configure them, check out the model architectures documentation.

Defining sublayers

Model architecture functions often accept sublayers as arguments, so that you can try substituting a different layer into the network. Depending on how the architecture function is structured, you might be able to define your network structure entirely through the config system, using layers that have already been defined. ​

In most neural network models for NLP, the most important parts of the network are what we refer to as the embed and encode steps. These steps together compute dense, context-sensitive representations of the tokens, and their combination forms a typical Tok2Vec layer:

### config.cfg (excerpt)
[components.tok2vec]
factory = "tok2vec"

[components.tok2vec.model]
@architectures = "spacy.Tok2Vec.v1"

[components.tok2vec.model.embed]
@architectures = "spacy.MultiHashEmbed.v1"
# ...

[components.tok2vec.model.encode]
@architectures = "spacy.MaxoutWindowEncoder.v1"
# ...

By defining these sublayers specifically, it becomes straightforward to swap out a sublayer for another one, for instance changing the first sublayer to a character embedding with the CharacterEmbed architecture:

### config.cfg (excerpt)
[components.tok2vec.model.embed]
@architectures = "spacy.CharacterEmbed.v1"
# ...

[components.tok2vec.model.encode]
@architectures = "spacy.MaxoutWindowEncoder.v1"
# ...

Most of spaCy's default architectures accept a tok2vec layer as a sublayer within the larger task-specific neural network. This makes it easy to switch between transformer, CNN, BiLSTM or other feature extraction approaches. The transformers documentation section shows an example of swapping out a model's standard tok2vec layer with a transformer. And if you want to define your own solution, all you need to do is register a Model[List[Doc], List[Floats2d]] architecture function, and you'll be able to try it out in any of the spaCy components. ​

Wrapping PyTorch, TensorFlow and other frameworks

Thinc allows you to wrap models written in other machine learning frameworks like PyTorch, TensorFlow and MXNet using a unified Model API. This makes it easy to use a model implemented in a different framework to power a component in your spaCy pipeline. For example, to wrap a PyTorch model as a Thinc Model, you can use Thinc's PyTorchWrapper:

from thinc.api import PyTorchWrapper

wrapped_pt_model = PyTorchWrapper(torch_model)

Let's use PyTorch to define a very simple neural network consisting of two hidden Linear layers with ReLU activation and dropout, and a softmax-activated output layer:

### PyTorch model
from torch import nn

torch_model = nn.Sequential(
    nn.Linear(width, hidden_width),
    nn.ReLU(),
    nn.Dropout2d(dropout),
    nn.Linear(hidden_width, nO),
    nn.ReLU(),
    nn.Dropout2d(dropout),
    nn.Softmax(dim=1)
)

The resulting wrapped Model can be used as a custom architecture as such, or can be a subcomponent of a larger model. For instance, we can use Thinc's chain combinator, which works like Sequential in PyTorch, to combine the wrapped model with other components in a larger network. This effectively means that you can easily wrap different components from different frameworks, and "glue" them together with Thinc:

from thinc.api import chain, with_array, PyTorchWrapper
from spacy.ml import CharacterEmbed

wrapped_pt_model = PyTorchWrapper(torch_model)
char_embed = CharacterEmbed(width, embed_size, nM, nC)
model = chain(char_embed, with_array(wrapped_pt_model))

In the above example, we have combined our custom PyTorch model with a character embedding layer defined by spaCy. CharacterEmbed returns a Model that takes a List[Doc] as input, and outputs a List[Floats2d]. To make sure that the wrapped PyTorch model receives valid inputs, we use Thinc's with_array helper.

You could also implement a model that only uses PyTorch for the transformer layers, and "native" Thinc layers to do fiddly input and output transformations and add on task-specific "heads", as efficiency is less of a consideration for those parts of the network.

Using wrapped models

To use our custom model including the PyTorch subnetwork, all we need to do is register the architecture using the architectures registry. This will assign the architecture a name so spaCy knows how to find it, and allows passing in arguments like hyperparameters via the config. The full example then becomes:

### Registering the architecture {highlight="9"}
from typing import List
from thinc.types import Floats2d
from thinc.api import Model, PyTorchWrapper, chain, with_array
import spacy
from spacy.tokens.doc import Doc
from spacy.ml import CharacterEmbed
from torch import nn

@spacy.registry.architectures("CustomTorchModel.v1")
def create_torch_model(
    nO: int,
    width: int,
    hidden_width: int,
    embed_size: int,
    nM: int,
    nC: int,
    dropout: float,
) -> Model[List[Doc], List[Floats2d]]:
    char_embed = CharacterEmbed(width, embed_size, nM, nC)
    torch_model = nn.Sequential(
        nn.Linear(width, hidden_width),
        nn.ReLU(),
        nn.Dropout2d(dropout),
        nn.Linear(hidden_width, nO),
        nn.ReLU(),
        nn.Dropout2d(dropout),
        nn.Softmax(dim=1)
    )
    wrapped_pt_model = PyTorchWrapper(torch_model)
    model = chain(char_embed, with_array(wrapped_pt_model))
    return model

The model definition can now be used in any existing trainable spaCy component, by specifying it in the config file. In this configuration, all required parameters for the various subcomponents of the custom architecture are passed in as settings via the config.

### config.cfg (excerpt) {highlight="5-5"}
[components.tagger]
factory = "tagger"

[components.tagger.model]
@architectures = "CustomTorchModel.v1"
nO = 50
width = 96
hidden_width = 48
embed_size = 2000
nM = 64
nC = 8
dropout = 0.2

Remember that it is best not to rely on any (hidden) default values, to ensure that training configs are complete and experiments fully reproducible.

Custom models with Thinc

Of course it's also possible to define the Model from the previous section entirely in Thinc. The Thinc documentation provides details on the various layers and helper functions available. Combinators can also be used to overload operators and a common usage pattern is to bind chain to >>. The "native" Thinc version of our simple neural network would then become:

from thinc.api import chain, with_array, Model, Relu, Dropout, Softmax
from spacy.ml import CharacterEmbed

char_embed = CharacterEmbed(width, embed_size, nM, nC)
with Model.define_operators({">>": chain}):
    layers = (
        Relu(hidden_width, width)
        >> Dropout(dropout)
        >> Relu(hidden_width, hidden_width)
        >> Dropout(dropout)
        >> Softmax(nO, hidden_width)
    )
    model = char_embed >> with_array(layers)

Note that Thinc layers define the output dimension (nO) as the first argument, followed (optionally) by the input dimension (nI). This is in contrast to how the PyTorch layers are defined, where in_features precedes out_features.

Shape inference in Thinc

It is not strictly necessary to define all the input and output dimensions for each layer, as Thinc can perform shape inference between sequential layers by matching up the output dimensionality of one layer to the input dimensionality of the next. This means that we can simplify the layers definition:

Diff

layers = (
    Relu(hidden_width, width)
    >> Dropout(dropout)
-   >> Relu(hidden_width, hidden_width)
+    >> Relu(hidden_width)
    >> Dropout(dropout)
-   >> Softmax(nO, hidden_width)
+   >> Softmax(nO)
)

with Model.define_operators({">>": chain}):
    layers = (
        Relu(hidden_width, width)
        >> Dropout(dropout)
        >> Relu(hidden_width)
        >> Dropout(dropout)
        >> Softmax(nO)
    )

Thinc can even go one step further and deduce the correct input dimension of the first layer, and output dimension of the last. To enable this functionality, you have to call Model.initialize with an input sample X and an output sample Y with the correct dimensions:

### Shape inference with initialization {highlight="3,7,10"}
with Model.define_operators({">>": chain}):
    layers = (
        Relu(hidden_width)
        >> Dropout(dropout)
        >> Relu(hidden_width)
        >> Dropout(dropout)
        >> Softmax()
    )
    model = char_embed >> with_array(layers)
    model.initialize(X=input_sample, Y=output_sample)

The built-in pipeline components in spaCy ensure that their internal models are always initialized with appropriate sample data. In this case, X is typically a List[Doc], while Y is typically a List[Array1d] or List[Array2d], depending on the specific task. This functionality is triggered when nlp.begin_training is called.

Dropout and normalization in Thinc

Many of the available Thinc layers allow you to define a dropout argument that will result in "chaining" an additional Dropout layer. Optionally, you can often specify whether or not you want to add layer normalization, which would result in an additional LayerNorm layer. That means that the following layers definition is equivalent to the previous:

with Model.define_operators({">>": chain}):
    layers = (
        Relu(hidden_width, dropout=dropout, normalize=False)
        >> Relu(hidden_width, dropout=dropout, normalize=False)
        >> Softmax()
    )
    model = char_embed >> with_array(layers)
    model.initialize(X=input_sample, Y=output_sample)

Create new trainable components