flairNLPGH-3496: Add OneClassClassifier model

flair/models/one_class_classification_model.py

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+from pathlib import Path
+from typing import Any, Dict, List, Optional, Tuple, Union, cast
+
+import numpy as np
+import torch
+from torch.utils.data import Dataset
+from tqdm import tqdm
+
+import flair
+from flair.data import Dictionary, Sentence, _iter_dataset
+from flair.datasets import DataLoader, FlairDatapointDataset
+from flair.embeddings import DocumentEmbeddings
+from flair.training_utils import store_embeddings
+
+
+class OneClassClassifier(flair.nn.Classifier[Sentence]):
+    """One Class Classification Model for tasks such as Anomaly Detection.
+
+    Task
+    ----
+    One Class Classification (OCC) tries to identify objects of a specific class amongst all objects, in contrast to
+    distinguishing between two or more classes.
+
+    Example:
+    -------
+    The model expects to be trained on a dataset in which every element has the same label_value, e.g. movie reviews
+    with the label POSITIVE.
+    During inference, one of two label_values will be added:
+    - In-class (e.g. another movie review) -> label_value="POSITIVE"
+    - Anything else (e.g. a wiki page)     -> label_value="<unk>"
+
+    Architecture
+    ------------
+    Reconstruction with autoencoder. The score is the reconstruction error from compressing and decompressing the
+    document embedding. A LOWER score indicates a HIGHER probability of being in-class. The threshold is
+    calculated as a high percentile of the score distribution of in-class elements from the dev set.
+
+    You must set the threshold after training by running `model.threshold = model.calculate_threshold(corpus.dev)`.
+    """
+
+    def __init__(
+        self,
+        embeddings: DocumentEmbeddings,
+        label_dictionary: Dictionary,
+        label_type: str,
+        encoding_dim: int = 128,
+        threshold: Optional[float] = None,
+    ) -> None:
+        """Initializes a OneClassClassifier.
+
+        Args:
+        embeddings: Embeddings to use during training and prediction
+        label_dictionary: The label to predict. Must contain exactly one class.
+        label_type: name of the annotation_layer to be predicted in case a corpus has multiple annotations
+        encoding_dim: The size of the compressed embedding
+        threshold: The score that separates in-class from out-of-class
+        """
+        super().__init__()
+        self.embeddings = embeddings
+        if len(label_dictionary) != 1:
+            raise ValueError(f"label_dictionary must have exactly 1 element: {label_dictionary}")
+        self.label_dictionary = label_dictionary
+        self.label_value = label_dictionary.get_items()[0]
+        self._label_type = label_type
+        self.encoding_dim = encoding_dim
+        self.threshold = threshold
+
+        embedding_dim = embeddings.embedding_length
+        self.encoder = torch.nn.Sequential(
+            torch.nn.Linear(embedding_dim, encoding_dim * 4),
+            torch.nn.LeakyReLU(True),
+            torch.nn.Linear(encoding_dim * 4, encoding_dim * 2),
+            torch.nn.LeakyReLU(True),
+            torch.nn.Linear(encoding_dim * 2, encoding_dim),
+            torch.nn.LeakyReLU(True),
+        )
+
+        self.decoder = torch.nn.Sequential(
+            torch.nn.Linear(encoding_dim, encoding_dim * 2),
+            torch.nn.LeakyReLU(True),
+            torch.nn.Linear(encoding_dim * 2, encoding_dim * 4),
+            torch.nn.LeakyReLU(True),
+            torch.nn.Linear(encoding_dim * 4, embedding_dim),
+            torch.nn.LeakyReLU(True),
+        )
+
+        self.cosine_sim = torch.nn.CosineSimilarity(dim=1)
+        self.to(flair.device)
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        x = self.encoder(x)
+        x = self.decoder(x)
+        return x
+
+    def forward_loss(self, sentences: List[Sentence]) -> Tuple[torch.Tensor, int]:
+        """Returns Tuple[scalar tensor, num examples]."""
+        if len(sentences) == 0:
+            return torch.tensor(0.0, dtype=torch.float, device=flair.device, requires_grad=True), 0
+        sentence_tensor = self._sentences_to_tensor(sentences)
+        reconstructed_sentence_tensor = self.forward(sentence_tensor)
+        return self._calculate_loss(reconstructed_sentence_tensor, sentence_tensor).sum(), len(sentences)
+
+    def predict(
+        self,
+        sentences: Union[List[Sentence], Sentence],
+        mini_batch_size: int = 32,
+        return_probabilities_for_all_classes: bool = False,
+        verbose: bool = False,
+        label_name: Optional[str] = None,
+        return_loss=False,
+        embedding_storage_mode="none",
+    ) -> Optional[torch.Tensor]:
+        """Predicts the class labels for the given sentences. The labels are directly added to the sentences.
+
+        Args:
+            sentences: list of sentences to predict
+            mini_batch_size: the amount of sentences that will be predicted within one batch
+            return_probabilities_for_all_classes: doesn't do anything for this class
+            verbose: set to True to display a progress bar
+            return_loss: set to True to return loss
+            label_name: set this to change the name of the label type that is predicted
+            embedding_storage_mode: default is 'none' which is the best is most cases.
+                Only set to 'cpu' or 'gpu' if you wish to not only predict, but also keep the generated embeddings in CPU or GPU memory respectively. 'gpu' to store embeddings in GPU memory.
+
+        Returns: None. If return_loss is set, returns a scalar tensor
+        """
+        if label_name is None:
+            label_name = self.label_type
+
+        with torch.no_grad():
+            # make sure it's a list
+            if not isinstance(sentences, list):
+                sentences = [sentences]
+
+            Sentence.set_context_for_sentences(cast(List[Sentence], sentences))
+
+            # filter empty sentences
+            sentences = [sentence for sentence in sentences if len(sentence) > 0]
+            if len(sentences) == 0:
+                return torch.tensor(0.0, requires_grad=True, device=flair.device) if return_loss else None
+
+            dataloader = DataLoader(
+                dataset=FlairDatapointDataset(sentences),
+                batch_size=mini_batch_size,
+            )
+            # progress bar for verbosity
+            if verbose:
+                dataloader = tqdm(dataloader, desc="Batch inference")
+
+            overall_loss = torch.zeros(1, device=flair.device)
+            for batch in dataloader:
+                # stop if all sentences are empty
+                if not batch:
+                    continue
+
+                sentence_tensor = self._sentences_to_tensor(batch)
+                reconstructed = self.forward(sentence_tensor)
+                loss_tensor = self._calculate_loss(reconstructed, sentence_tensor)
+
+                for sentence, loss in zip(batch, loss_tensor.tolist()):
+                    sentence.remove_labels(label_name)
+                    label_value = self.label_value if self.threshold is not None and loss < self.threshold else "<unk>"
+                    sentence.add_label(typename=label_name, value=label_value, score=loss)
+
+                overall_loss += loss_tensor.sum()
+                store_embeddings(batch, storage_mode=embedding_storage_mode)
+
+        return overall_loss if return_loss else None
+
+    @property
+    def label_type(self) -> str:
+        return self._label_type
+
+    def _sentences_to_tensor(self, sentences: List[Sentence]) -> torch.Tensor:
+        self.embeddings.embed(sentences)
+        return torch.stack([sentence.embedding for sentence in sentences])
+
+    def _calculate_loss(self, predicted: torch.Tensor, labels: torch.Tensor) -> torch.Tensor:
+        """Return cosine similarity loss.
+
+        Args:
+            predicted: tensor of shape (batch_size, embedding_size)
+            labels: tensor of shape (batch_size, embedding_size)
+
+        Returns:
+            tensor of shape (batch_size)
+        """
+        if labels.size(0) == 0:
+            return torch.tensor(0.0, requires_grad=True, device=flair.device)
+
+        return 1 - self.cosine_sim(predicted, labels)
+
+    def _get_state_dict(self):
+        """Returns the state dictionary for this model."""
+        model_state = {
+            **super()._get_state_dict(),
+            "embeddings": self.embeddings.save_embeddings(use_state_dict=False),
+            "label_dictionary": self.label_dictionary,
+            "label_type": self.label_type,
+            "encoding_dim": self.encoding_dim,
+            "threshold": self.threshold,
+        }
+
+        return model_state
+
+    @classmethod
+    def _init_model_with_state_dict(cls, state, **kwargs):
+        return super()._init_model_with_state_dict(
+            state,
+            embeddings=state.get("embeddings"),
+            label_dictionary=state.get("label_dictionary"),
+            label_type=state.get("label_type"),
+            encoding_dim=state.get("encoding_dim"),
+            threshold=state.get("threshold"),
+            **kwargs,
+        )
+
+    @classmethod
+    def load(cls, model_path: Union[str, Path, Dict[str, Any]]) -> "OneClassClassifier":
+        from typing import cast
+
+        return cast("OneClassClassifier", super().load(model_path=model_path))
+
+    def calculate_threshold(self, dataset: Dataset[Sentence], quantile=0.995) -> float:
+        """Determine the score threshold to consider a Sentence in-class.
+
+        This implementation returns the score at which `quantile` of `dataset` will be considered in-class. Intended
+        for use-cases desiring high-recall.
+        """
+
+        def score(sentence: Sentence) -> float:
+            sentence_tensor = self._sentences_to_tensor([sentence])
+            reconstructed = self.forward(sentence_tensor)
+            loss_tensor = self._calculate_loss(reconstructed, sentence_tensor)
+            return loss_tensor.tolist()[0]
+
+        scores = [
+            score(sentence)
+            for sentence in _iter_dataset(dataset)
+            if sentence.get_labels(self.label_type)[0].value == self.label_value
+        ]
+        threshold = np.quantile(scores, quantile)
+        return threshold

tests/models/test_one_class_classifier.py

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+import pytest
+
+import flair
+from flair.embeddings import TransformerDocumentEmbeddings
+from flair.models.one_class_classification_model import OneClassClassifier
+from flair.trainers import ModelTrainer
+from tests.model_test_utils import BaseModelTest
+
+
+class TestOneClassClassifier(BaseModelTest):
+    model_cls = OneClassClassifier
+    train_label_type = "topic"
+    training_args = {
+        "max_epochs": 2,
+    }
+
+    @pytest.fixture()
+    def corpus(self, tasks_base_path):
+        label_type = "topic"
+        corpus = flair.datasets.ClassificationCorpus(tasks_base_path / "imdb", label_type=label_type)
+        corpus._train = [x for x in corpus.train if x.get_labels(label_type)[0].value == "POSITIVE"]
+        return corpus
+
+    @pytest.fixture()
+    def embeddings(self):
+        return TransformerDocumentEmbeddings(model="distilbert-base-uncased", layers="-1", fine_tune=True)
+
+    @pytest.mark.integration()
+    def test_train_load_use_one_class_classifier(self, results_base_path, corpus, example_sentence, embeddings):
+        label_dict = corpus.make_label_dictionary(label_type=self.train_label_type)
+
+        model = self.model_cls(embeddings=embeddings, label_dictionary=label_dict, label_type=self.train_label_type)
+        trainer = ModelTrainer(model, corpus)
+
+        trainer.train(results_base_path, shuffle=False, **self.training_args)
+
+        del trainer, model, label_dict, corpus
+        loaded_model = self.model_cls.load(results_base_path / "final-model.pt")
+
+        loaded_model.predict(example_sentence)
+        loaded_model.predict([example_sentence, self.empty_sentence])
+        loaded_model.predict([self.empty_sentence])
+
+        assert example_sentence.get_labels(self.train_label_type)[0].value in {"POSITIVE", "<unk>"}