CoSENTLoss.py

import torch
from torch import nn, Tensor
from typing import Iterable, Dict
from ..SentenceTransformer import SentenceTransformer
from .. import util


class CoSENTLoss(nn.Module):
    def __init__(self, model: SentenceTransformer, scale: float = 20.0, similarity_fct=util.pairwise_cos_sim):
        """
        This class implements CoSENT (Cosine Sentence) loss.
        It expects that each of the InputExamples consists of a pair of texts and a float valued label, representing
        the expected similarity score between the pair.

        It computes the following loss function:

        ``loss = logsum(1+exp(s(k,l)-s(i,j))+exp...)``, where ``(i,j)`` and ``(k,l)`` are any of the input pairs in the
        batch such that the expected similarity of ``(i,j)`` is greater than ``(k,l)``. The summation is over all possible
        pairs of input pairs in the batch that match this condition.

        Anecdotal experiments show that this loss function produces a more powerful training signal than :class:`CosineSimilarityLoss`,
        resulting in faster convergence and a final model with superior performance. Consequently, CoSENTLoss may be used
        as a drop-in replacement for :class:`CosineSimilarityLoss` in any training script.

        :param model: SentenceTransformerModel
        :param similarity_fct: Function to compute the PAIRWISE similarity between embeddings. Default is ``util.pairwise_cos_sim``.
        :param scale: Output of similarity function is multiplied by scale value. Represents the inverse temperature.

        References:
            - For further details, see: https://kexue.fm/archives/8847

        Requirements:
            - Sentence pairs with corresponding similarity scores in range of the similarity function. Default is [-1,1].

        Relations:
            - :class:`AnglELoss` is CoSENTLoss with ``pairwise_angle_sim`` as the metric, rather than ``pairwise_cos_sim``.
            - :class:`CosineSimilarityLoss` seems to produce a weaker training signal than CoSENTLoss. In our experiments, CoSENTLoss is recommended.

        Inputs:
            +--------------------------------+------------------------+
            | Texts                          | Labels                 |
            +================================+========================+
            | (sentence_A, sentence_B) pairs | float similarity score |
            +--------------------------------+------------------------+

        Example:
            ::

                from sentence_transformers import SentenceTransformer, losses
                from sentence_transformers.readers import InputExample

                model = SentenceTransformer('bert-base-uncased')
                train_examples = [InputExample(texts=['My first sentence', 'My second sentence'], label=1.0),
                        InputExample(texts=['My third sentence', 'Unrelated sentence'], label=0.3)]

                train_dataloader = DataLoader(train_examples, shuffle=True, batch_size=train_batch_size)
                train_loss = losses.CoSENTLoss(model=model)
        """
        super(CoSENTLoss, self).__init__()
        self.model = model
        self.similarity_fct = similarity_fct
        self.scale = scale

    def forward(self, sentence_features: Iterable[Dict[str, Tensor]], labels: Tensor):
        embeddings = [self.model(sentence_feature)["sentence_embedding"] for sentence_feature in sentence_features]

        scores = self.similarity_fct(embeddings[0], embeddings[1])
        scores = scores * self.scale
        scores = scores[:, None] - scores[None, :]

        # label matrix indicating which pairs are relevant
        labels = labels[:, None] < labels[None, :]
        labels = labels.float()

        # mask out irrelevant pairs so they are negligible after exp()
        scores = scores - (1 - labels) * 1e12

        # append a zero as e^0 = 1
        scores = torch.cat((torch.zeros(1).to(scores.device), scores.view(-1)), dim=0)
        loss = torch.logsumexp(scores, dim=0)

        return loss

    def get_config_dict(self):
        return {"scale": self.scale, "similarity_fct": self.similarity_fct.__name__}