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315 lines
12 KiB
Markdown
315 lines
12 KiB
Markdown
---
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title: Layers and Model Architectures
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teaser: Power spaCy components with custom neural networks
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menu:
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- ['Type Signatures', 'type-sigs']
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- ['Swapping Architectures', 'swap-architectures']
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- ['PyTorch & TensorFlow', 'frameworks']
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- ['Thinc Models', 'thinc']
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- ['Trainable Components', 'components']
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next: /usage/projects
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---
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> #### Example
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>
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> ```python
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> from thinc.api import Model, chain
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>
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> @spacy.registry.architectures.register("model.v1")
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> def build_model(width: int, classes: int) -> Model:
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> tok2vec = build_tok2vec(width)
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> output_layer = build_output_layer(width, classes)
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> model = chain(tok2vec, output_layer)
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> return model
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> ```
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A **model architecture** is a function that wires up a
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[Thinc `Model`](https://thinc.ai/docs/api-model) instance. It describes the
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neural network that is run internally as part of a component in a spaCy
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pipeline. To define the actual architecture, you can implement your logic in
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Thinc directly, or you can use Thinc as a thin wrapper around frameworks such as
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PyTorch, TensorFlow and MXNet. Each Model can also be used as a sublayer of a
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larger network, allowing you to freely combine implementations from different
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frameworks into one `Thinc` Model.
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spaCy's built-in components require a `Model` instance to be passed to them via
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the config system. To change the model architecture of an existing component,
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you just need to [**update the config**](#swap-architectures) so that it refers
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to a different registered function. Once the component has been created from
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this config, you won't be able to change it anymore. The architecture is like a
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recipe for the network, and you can't change the recipe once the dish has
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already been prepared. You have to make a new one.
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```ini
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### config.cfg (excerpt)
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[components.tagger]
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factory = "tagger"
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[components.tagger.model]
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@architectures = "model.v1"
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width = 512
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classes = 16
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```
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## Type signatures {#type-sigs}
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> #### Example
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>
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> ```python
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> from typing import List
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> from thinc.api import Model, chain
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> from thinc.types import Floats2d
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> def chain_model(
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> tok2vec: Model[List[Doc], List[Floats2d]],
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> layer1: Model[List[Floats2d], Floats2d],
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> layer2: Model[Floats2d, Floats2d]
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> ) -> Model[List[Doc], Floats2d]:
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> model = chain(tok2vec, layer1, layer2)
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> return model
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> ```
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The Thinc `Model` class is a **generic type** that can specify its input and
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output types. Python uses a square-bracket notation for this, so the type
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~~Model[List, Dict]~~ says that each batch of inputs to the model will be a
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list, and the outputs will be a dictionary. You can be even more specific and
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write for instance~~Model[List[Doc], Dict[str, float]]~~ to specify that the
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model expects a list of [`Doc`](/api/doc) objects as input, and returns a
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dictionary mapping of strings to floats. Some of the most common types you'll
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see are:
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| Type | Description |
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| ------------------ | ---------------------------------------------------------------------------------------------------- |
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| ~~List[Doc]~~ | A batch of [`Doc`](/api/doc) objects. Most components expect their models to take this as input. |
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| ~~Floats2d~~ | A two-dimensional `numpy` or `cupy` array of floats. Usually 32-bit. |
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| ~~Ints2d~~ | A two-dimensional `numpy` or `cupy` array of integers. Common dtypes include uint64, int32 and int8. |
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| ~~List[Floats2d]~~ | A list of two-dimensional arrays, generally with one array per `Doc` and one row per token. |
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| ~~Ragged~~ | A container to handle variable-length sequence data in an unpadded contiguous array. |
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| ~~Padded~~ | A container to handle variable-length sequence data in a padded contiguous array. |
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The model type signatures help you figure out which model architectures and
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components can **fit together**. For instance, the
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[`TextCategorizer`](/api/textcategorizer) class expects a model typed
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~~Model[List[Doc], Floats2d]~~, because the model will predict one row of
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category probabilities per [`Doc`](/api/doc). In contrast, the
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[`Tagger`](/api/tagger) class expects a model typed ~~Model[List[Doc],
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List[Floats2d]]~~, because it needs to predict one row of probabilities per
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token.
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There's no guarantee that two models with the same type signature can be used
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interchangeably. There are many other ways they could be incompatible. However,
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if the types don't match, they almost surely _won't_ be compatible. This little
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bit of validation goes a long way, especially if you
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[configure your editor](https://thinc.ai/docs/usage-type-checking) or other
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tools to highlight these errors early. The config file is also validated at the
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beginning of training, to verify that all the types match correctly.
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<Accordion title="Tip: Static type checking in your editor">
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If you're using a modern editor like Visual Studio Code, you can
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[set up `mypy`](https://thinc.ai/docs/usage-type-checking#install) with the
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custom Thinc plugin and get live feedback about mismatched types as you write
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code.
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[](https://thinc.ai/docs/usage-type-checking#linting)
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</Accordion>
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## Swapping model architectures {#swap-architectures}
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If no model is specified for the [`TextCategorizer`](/api/textcategorizer), the
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[TextCatEnsemble](/api/architectures#TextCatEnsemble) architecture is used by
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default. This architecture combines a simple bag-of-words model with a neural
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network, usually resulting in the most accurate results, but at the cost of
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speed. The config file for this model would look something like this:
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```ini
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### config.cfg (excerpt)
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[components.textcat]
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factory = "textcat"
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labels = []
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[components.textcat.model]
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@architectures = "spacy.TextCatEnsemble.v1"
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exclusive_classes = false
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pretrained_vectors = null
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width = 64
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conv_depth = 2
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embed_size = 2000
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window_size = 1
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ngram_size = 1
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dropout = 0
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nO = null
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```
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spaCy has two additional built-in `textcat` architectures, and you can easily
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use those by swapping out the definition of the textcat's model. For instance,
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to use the simple and fast bag-of-words model
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[TextCatBOW](/api/architectures#TextCatBOW), you can change the config to:
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```ini
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### config.cfg (excerpt) {highlight="6-10"}
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[components.textcat]
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factory = "textcat"
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labels = []
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[components.textcat.model]
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@architectures = "spacy.TextCatBOW.v1"
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exclusive_classes = false
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ngram_size = 1
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no_output_layer = false
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nO = null
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```
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For details on all pre-defined architectures shipped with spaCy and how to
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configure them, check out the [model architectures](/api/architectures)
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documentation.
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### Defining sublayers {#sublayers}
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Model architecture functions often accept **sublayers as arguments**, so that
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you can try **substituting a different layer** into the network. Depending on
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how the architecture function is structured, you might be able to define your
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network structure entirely through the [config system](/usage/training#config),
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using layers that have already been defined.
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In most neural network models for NLP, the most important parts of the network
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are what we refer to as the
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[embed and encode](https://explosion.ai/blog/deep-learning-formula-nlp) steps.
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These steps together compute dense, context-sensitive representations of the
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tokens, and their combination forms a typical
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[`Tok2Vec`](/api/architectures#Tok2Vec) layer:
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```ini
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### config.cfg (excerpt)
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[components.tok2vec]
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factory = "tok2vec"
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[components.tok2vec.model]
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@architectures = "spacy.Tok2Vec.v1"
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[components.tok2vec.model.embed]
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@architectures = "spacy.MultiHashEmbed.v1"
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# ...
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[components.tok2vec.model.encode]
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@architectures = "spacy.MaxoutWindowEncoder.v1"
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# ...
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```
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By defining these sublayers specifically, it becomes straightforward to swap out
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a sublayer for another one, for instance changing the first sublayer to a
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character embedding with the [CharacterEmbed](/api/architectures#CharacterEmbed)
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architecture:
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```ini
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### config.cfg (excerpt)
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[components.tok2vec.model.embed]
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@architectures = "spacy.CharacterEmbed.v1"
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# ...
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[components.tok2vec.model.encode]
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@architectures = "spacy.MaxoutWindowEncoder.v1"
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# ...
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```
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Most of spaCy's default architectures accept a `tok2vec` layer as a sublayer
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within the larger task-specific neural network. This makes it easy to **switch
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between** transformer, CNN, BiLSTM or other feature extraction approaches. The
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[transformers documentation](/usage/embeddings-transformers#training-custom-model)
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section shows an example of swapping out a model's standard `tok2vec` layer with
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a transformer. And if you want to define your own solution, all you need to do
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is register a ~~Model[List[Doc], List[Floats2d]]~~ architecture function, and
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you'll be able to try it out in any of the spaCy components.
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## Wrapping PyTorch, TensorFlow and other frameworks {#frameworks}
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Thinc allows you to [wrap models](https://thinc.ai/docs/usage-frameworks)
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written in other machine learning frameworks like PyTorch, TensorFlow and MXNet
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using a unified [`Model`](https://thinc.ai/docs/api-model) API.
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For example, let's use Pytorch to define a very simple Neural network consisting
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of two hidden `Linear` layers with `ReLU` activation and dropout, and a
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softmax-activated output layer.
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```python
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from torch import nn
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torch_model = nn.Sequential(
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nn.Linear(width, hidden_width),
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nn.ReLU(),
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nn.Dropout2d(dropout),
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nn.Linear(hidden_width, nO),
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nn.ReLU(),
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nn.Dropout2d(dropout),
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nn.Softmax(dim=1)
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)
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```
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This PyTorch model can be wrapped as a Thinc `Model` by using Thinc's `PyTorchWrapper`:
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```python
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from thinc.api import PyTorchWrapper
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wrapped_pt_model = PyTorchWrapper(torch_model)
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```
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The resulting wrapped `Model` can be used as a **custom architecture** as such, or
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can be a **subcomponent of a larger model**. For instance, we can use Thinc's
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[`chain`](https://thinc.ai/docs/api-layers#chain)
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combinator, which works like `Sequential` in PyTorch,
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to combine the wrapped model with other components in a larger network.
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This effectively means that you can easily wrap different components
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from different frameworks, and "glue" them together with Thinc:
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```python
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from thinc.api import chain, with_array
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from spacy.ml import CharacterEmbed
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embed = CharacterEmbed(width, embed_size, nM, nC)
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model = chain(embed, with_array(wrapped_pt_model))
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```
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In the above example, we have combined our custom PyTorch model with a
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character embedding layer defined by spaCy.
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[CharacterEmbed](/api/architectures#CharacterEmbed) returns a
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`Model` that takes a `List[Doc]` as input, and outputs a `List[Floats2d]`.
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To make sure that the wrapped Pytorch model receives valid inputs, we use Thinc's
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[`with_array`](https://thinc.ai/docs/api-layers#with_array) helper.
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As another example, you could have a model where you use PyTorch just for
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the transformer layers, and use "native" Thinc layers to do fiddly input and
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output transformations and add on task-specific "heads", as efficiency is less
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of a consideration for those parts of the network.
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<!-- TODO: custom tagger implemented in PyTorch, wrapped as Thinc model, link off to project (with notebook?) -->
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## Implementing models in Thinc {#thinc}
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<!-- TODO: use same example as above, custom tagger, but implemented in Thinc, link off to Thinc docs where appropriate -->
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## Models for trainable components {#components}
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<!-- TODO:
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- Interaction with `predict`, `get_loss` and `set_annotations`
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- Initialization life-cycle with `begin_training`.
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Example: relation extraction component (implemented as project template)
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-->
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```python
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def update(self, examples):
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docs = [ex.predicted for ex in examples]
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refs = [ex.reference for ex in examples]
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predictions, backprop = self.model.begin_update(docs)
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gradient = self.get_loss(predictions, refs)
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backprop(gradient)
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def __call__(self, doc):
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predictions = self.model([doc])
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self.set_annotations(predictions)
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```
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