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import torch
from torch import nn, Tensor
from typing import Union, Tuple, List, Iterable, Dict
from .BatchHardTripletLoss import BatchHardTripletLoss, BatchHardTripletLossDistanceFunction
from sentence_transformers.SentenceTransformer import SentenceTransformer
class BatchHardSoftMarginTripletLoss(BatchHardTripletLoss):
"""
BatchHardSoftMarginTripletLoss takes a batch with (label, sentence) pairs and computes the loss for all possible, valid
triplets, i.e., anchor and positive must have the same label, anchor and negative a different label. The labels
must be integers, with same label indicating sentences from the same class. You train dataset
must contain at least 2 examples per label class. The margin is computed automatically.
Source: https://github.com/NegatioN/OnlineMiningTripletLoss/blob/master/online_triplet_loss/losses.py
Paper: In Defense of the Triplet Loss for Person Re-Identification, https://arxiv.org/abs/1703.07737
Blog post: https://omoindrot.github.io/triplet-loss
:param model: SentenceTransformer model
:param distance_metric: Function that returns a distance between two emeddings. The class SiameseDistanceMetric contains pre-defined metrices that can be used
Example::
from sentence_transformers import SentenceTransformer, SentencesDataset, LoggingHandler, losses
from sentence_transformers.readers import InputExample
model = SentenceTransformer('distilbert-base-nli-mean-tokens')
train_examples = [InputExample(texts=['Sentence from class 0'], label=0), InputExample(texts=['Another sentence from class 0'], label=0),
InputExample(texts=['Sentence from class 1'], label=1), InputExample(texts=['Sentence from class 2'], label=2)]
train_dataset = SentencesDataset(train_examples, model)
train_dataloader = DataLoader(train_dataset, shuffle=True, batch_size=train_batch_size)
train_loss = losses.BatchHardSoftMarginTripletLoss(model=model)
"""
def __init__(self, model: SentenceTransformer, distance_metric=BatchHardTripletLossDistanceFunction.eucledian_distance):
super(BatchHardSoftMarginTripletLoss, self).__init__(model)
self.sentence_embedder = model
self.distance_metric = distance_metric
def forward(self, sentence_features: Iterable[Dict[str, Tensor]], labels: Tensor):
rep = self.sentence_embedder(sentence_features[0])['sentence_embedding']
return self.batch_hard_triplet_soft_margin_loss(labels, rep)
# Hard Triplet Loss with Soft Margin
# Paper: In Defense of the Triplet Loss for Person Re-Identification, https://arxiv.org/abs/1703.07737
def batch_hard_triplet_soft_margin_loss(self, labels: Tensor, embeddings: Tensor) -> Tensor:
"""Build the triplet loss over a batch of embeddings.
For each anchor, we get the hardest positive and hardest negative to form a triplet.
Args:
labels: labels of the batch, of size (batch_size,)
embeddings: tensor of shape (batch_size, embed_dim)
squared: Boolean. If true, output is the pairwise squared euclidean distance matrix.
If false, output is the pairwise euclidean distance matrix.
Returns:
Label_Sentence_Triplet: scalar tensor containing the triplet loss
"""
# Get the pairwise distance matrix
pairwise_dist = self.distance_metric(embeddings)
# For each anchor, get the hardest positive
# First, we need to get a mask for every valid positive (they should have same label)
mask_anchor_positive = BatchHardTripletLoss.get_anchor_positive_triplet_mask(labels).float()
# We put to 0 any element where (a, p) is not valid (valid if a != p and label(a) == label(p))
anchor_positive_dist = mask_anchor_positive * pairwise_dist
# shape (batch_size, 1)
hardest_positive_dist, _ = anchor_positive_dist.max(1, keepdim=True)
# For each anchor, get the hardest negative
# First, we need to get a mask for every valid negative (they should have different labels)
mask_anchor_negative = BatchHardTripletLoss.get_anchor_negative_triplet_mask(labels).float()
# We add the maximum value in each row to the invalid negatives (label(a) == label(n))
max_anchor_negative_dist, _ = pairwise_dist.max(1, keepdim=True)
anchor_negative_dist = pairwise_dist + max_anchor_negative_dist * (1.0 - mask_anchor_negative)
# shape (batch_size,)
hardest_negative_dist, _ = anchor_negative_dist.min(1, keepdim=True)
# Combine biggest d(a, p) and smallest d(a, n) into final triplet loss with soft margin
#tl = hardest_positive_dist - hardest_negative_dist + margin
#tl[tl < 0] = 0
tl = torch.log1p(torch.exp(hardest_positive_dist - hardest_negative_dist))
triplet_loss = tl.mean()
return triplet_loss
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