Contrastive Learning

SwaV, or Swapping Assignments Between Views, is a self-supervised learning approach that takes advantage of contrastive methods without requiring to compute pairwise comparisons. Specifically, it simultaneously clusters the data while enforcing consistency between cluster assignments produced for different augmentations (or views) of the same image, instead of comparing features directly as in contrastive learning. Simply put, SwaV uses a swapped prediction mechanism where we predict the cluster assignment of a view from the representation of another view.

SimCSE is a contrastive learning framework for generating sentence embeddings. It utilizes an unsupervised approach, which takes an input sentence and predicts itself in contrastive objective, with only standard dropout used as noise. The authors find that dropout acts as minimal “data augmentation” of hidden representations, while removing it leads to a representation collapse. Afterwards a supervised approach is used, which incorporates annotated pairs from natural language inference datasets into the contrastive framework, by using “entailment” pairs as positives and “contradiction

COLA is a self-supervised pre-training approach for learning a general-purpose representation of audio. It is based on contrastive learning: it learns a representation which assigns high similarity to audio segments extracted from the same recording while assigning lower similarity to segments from different recordings.

MoCo v2 is an improved version of the Momentum Contrast self-supervised learning algorithm. Motivated by the findings presented in the SimCLR paper, authors: - Replace the 1-layer fully connected layer with a 2-layer MLP head with ReLU for the unsupervised training stage. - Include blur augmentation. - Use cosine learning rate schedule. These modifications enable MoCo to outperform the state-of-the-art SimCLR with a smaller batch size and fewer epochs.

CMCL, or Crossmodal Contrastive Learning, is a method for unifying visual and textual representations into the same semantic space based on a large-scale corpus of image collections, text corpus and image-text pairs. The CMCL aligns the visual representations and textual representations, and unifies them into the same semantic space based on image-text pairs. As shown in the Figure, to facilitate different levels of semantic alignment between vision and language, a series of text rewriting techniques are utilized to improve the diversity of cross-modal information. Specifically, for an image-text pair, various positive examples and hard negative examples can be obtained by rewriting the original caption at different levels. Moreover, to incorporate more background information from the single-modal data, text and image retrieval are also applied to augment each image-text pair with various related texts and images. The positive pairs, negative pairs, related images and texts are learned jointly by CMCL. In this way, the model can effectively unify different levels of visual and textual representations into the same semantic space, and incorporate more single-modal knowledge to enhance each other.

ReLIC, or Representation Learning via Invariant Causal Mechanisms, is a self-supervised learning objective that enforces invariant prediction of proxy targets across augmentations through an invariance regularizer which yields improved generalization guarantees. We can write the objective as: $$ \underset{X}{\mathbb{E}} \underset{\sim\_{l k}, a\_{q \mathcal{A}}}{\mathbb{E}} \sum_{b \in\left\(a\_{l k}, a\_{q t}\right\)} \mathcal{L}\_{b}\left(Y^{R}, f(X)\right) \text { s.t. } K L\left(p^{d o\left(a\_{l k}\right)}\left(Y^{R} \mid f(X)\right), p^{d o\left(a\_{q t}\right)}\left(Y^{R} \mid f(X)\right)\right) \leq \rho $$ where $\mathcal{L}$ is the proxy task loss and $K L$ is the Kullback-Leibler (KL) divergence. Note that any distance measure on distributions can be used in place of the KL divergence. Concretely, as proxy task we associate to every datapoint $x\_{i}$ the label $y\_{i}^{R}=i$. This corresponds to the instance discrimination task, commonly used in contrastive learning. We take pairs of points $\left(x\_{i}, x\_{j}\right)$ to compute similarity scores and use pairs of augmentations $a\_{l k}=\left(a\_{l}, a\_{k}\right) \in$ $\mathcal{A} \times \mathcal{A}$ to perform a style intervention. Given a batch of samples $\left\(x\_{i}\right\)\_{i=1}^{N} \sim \mathcal{D}$, we use $$ p^{d o\left(a\_{l k}\right)}\left(Y^{R}=j \mid f\left(x\_{i}\right)\right) \propto \exp \left(\phi\left(f\left(x\_{i}^{a\_{l}}\right), h\left(x\_{j}^{a\_{k}}\right)\right) / \tau\right) $$ with $x^{a}$ data augmented with $a$ and $\tau$ a softmax temperature parameter. We encode $f$ using a neural network and choose $h$ to be related to $f$, e.g. $h=f$ or as a network with an exponential moving average of the weights of $f$ (e.g. target networks). To compare representations we use the function $\phi\left(f\left(x\_{i}\right), h\left(x\_{j}\right)\right)=\left\langle g\left(f\left(x\_{i}\right)\right), g\left(h\left(x\_{j}\right)\right)\right\rangle$ where $g$ is a fully-connected neural network often called the critic. Combining these pieces, we learn representations by minimizing the following objective over the full set of data $x\_{i} \in \mathcal{D}$ and augmentations $a_{l k} \in \mathcal{A} \times \mathcal{A}$ $$ -\sum_{i=1}^{N} \sum\_{a\_{l k}} \log \frac{\exp \left(\phi\left(f\left(x\_{i}^{a_{l}}\right), h\left(x\_{i}^{a\_{k}}\right)\right) / \tau\right)}{\sum\_{m=1}^{M} \exp \left(\phi\left(f\left(x\_{i}^{a\_{l}}\right), h\left(x\_{m}^{a\_{k}}\right)\right) / \tau\right)}+\alpha \sum\_{a\_{l k}, a\_{q t}} K L\left(p^{d o\left(a\_{l k}\right)}, p^{d o\left(a\_{q t}\right)}\right) $$ with $M$ the number of points we use to construct the contrast set and $\alpha$ the weighting of the invariance penalty. The shorthand $p^{d o(a)}$ is used for $p^{d o(a)}\left(Y^{R}=j \mid f\left(x\_{i}\right)\right)$. The Figure shows a schematic of the RELIC objective.

Dense Contrastive Learning is a self-supervised learning method for dense prediction tasks. It implements self-supervised learning by optimizing a pairwise contrastive (dis)similarity loss at the pixel level between two views of input images. Contrasting with regular contrastive loss, the contrastive loss is computed between the single feature vectors outputted by the global projection head, at the level of global feature, while the dense contrastive loss is computed between the dense feature vectors outputted by the dense projection head, at the level of local feature.

Pretext-Invariant Representation Learning (PIRL, pronounced as “pearl”) learns invariant representations based on pretext tasks. PIRL is used with a commonly used pretext task that involves solving jigsaw puzzles. Specifically, PIRL constructs image representations that are similar to the representation of transformed versions of the same image and different from the representations of other images.

Mirror-BERT converts pretrained language models into effective universal text encoders without any supervision, in 20-30 seconds. It is an extremely simple, fast, and effective contrastive learning technique. It relies on fully identical or slightly modified string pairs as positive (i.e., synonymous) fine-tuning examples, and aims to maximise their similarity during identity fine-tuning.

In this work, we present a conceptually simple and effective method to train a strong bilingual multimodal representation model. Starting from the pretrained multimodal representation model CLIP released by OpenAI, we switched its text encoder with a pretrained multilingual text encoder XLM-R, and aligned both languages and image representations by a two-stage training schema consisting of teacher learning and contrastive learning. We validate our method through evaluations of a wide range of tasks. We set new state-of-the-art performances on a bunch of tasks including ImageNet-CN, Flicker30k- CN, and COCO-CN. Further, we obtain very close performances with CLIP on almost all tasks, suggesting that one can simply alter the text encoder in CLIP for extended capabilities such as multilingual understanding. Our models and code are available at https://github.com/FlagAI-Open/FlagAI.

Contrastive Multiview Coding (CMC) is a self-supervised learning approach, based on CPC, that learns representations that capture information shared between multiple sensory views. The core idea is to set an anchor view and the sample positive and negative data points from the other view and maximise agreement between positive pairs in learning from two views. Contrastive learning is used to build the embedding.

BigBiGAN is a type of BiGAN with a BigGAN image generator. The authors initially used ResNet as a baseline for the encoder $\mathcal{E}$ followed by a 4-layer MLP with skip connections, but they experimented with RevNets and found they outperformed with increased network width, so opted for this type of encoder for the final architecture.

Graph Contrastive Coding is a self-supervised graph neural network pre-training framework to capture the universal network topological properties across multiple networks. GCC's pre-training task is designed as subgraph instance discrimination in and across networks and leverages contrastive learning to empower graph neural networks to learn the intrinsic and transferable structural representations.

SCCL, or Supporting Clustering with Contrastive Learning, is a framework to leverage contrastive learning to promote better separation in unsupervised clustering. It combines the top-down clustering with the bottom-up instance-wise contrastive learning to achieve better inter-cluster distance and intra-cluster distance. During training, we jointly optimize a clustering loss over the original data instances and an instance-wise contrastive loss over the associated augmented pairs.

UNIMO is a multi-modal pre-training architecture that can effectively adapt to both single modal and multimodal understanding and generation tasks. UNIMO learns visual representations and textual representations simultaneously, and unifies them into the same semantic space via cross-modal contrastive learning (CMCL) based on a large-scale corpus of image collections, text corpus and image-text pairs. The CMCL aligns the visual representation and textual representation, and unifies them into the same semantic space based on image-text pairs.

Proposes a two-tower pre-training model called BriVL within the cross-modal contrastive learning framework. A cross-modal pre-training model is defined based on the image-text retrieval task. The main goal is thus to learn two encoders that can embed image and text samples into the same space for effective image-text retrieval. To enforce such cross-modal embedding learning, we introduce contrastive learning with the InfoNCE loss into the BriVL model. Given text embedding, the learning objective aims to find the best image embedding from a batch of image embeddings. Similarly, for a given image embedding, the learning objective is to find the best text embedding from a batch of text embeddings. The pre-training model learns a cross-modal embedding space by jointly training the image and text encoders to maximize the cosine similarity of the image and text embeddings of the true pair for each sample in the batch while minimizing the cosine similarity of the embeddings of the other incorrect pairs.

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3D ResNet-RS is an architecture and scaling strategy for 3D ResNets for video recognition. The key additions are: - 3D ResNet-D stem: The ResNet-D stem is adapted to 3D inputs by using three consecutive 3D convolutional layers. The first convolutional layer employs a temporal kernel size of 5 while the remaining two convolutional layers employ a temporal kernel size of 1. - 3D Squeeze-and-Excitation: Squeeze-and-Excite is adapted to spatio-temporal inputs by using a 3D global average pooling operation for the squeeze operation. A SE ratio of 0.25 is applied in each 3D bottleneck block for all experiments. - Self-gating: A self-gating module is used in each 3D bottleneck block after the SE module.

Contrastive Video Representation Learning, or CVRL, is a self-supervised contrastive learning framework for learning spatiotemporal visual representations from unlabeled videos. Representations are learned using a contrastive loss, where two clips from the same short video are pulled together in the embedding space, while clips from different videos are pushed away. Data augmentations are designed involving spatial and temporal cues. Concretely, a temporally consistent spatial augmentation method is used to impose strong spatial augmentations on each frame of the video while maintaining the temporal consistency across frames. A sampling-based temporal augmentation method is also used to avoid overly enforcing invariance on clips that are distant in time. End-to-end, from a raw video, we first sample a temporal interval from a monotonically decreasing distribution. The temporal interval represents the number of frames between the start points of two clips, and we sample two clips from a video according to this interval. Afterwards we apply a temporally consistent spatial augmentation to each of the clips and feed them into a 3D backbone with an MLP head. The contrastive loss is used to train the network to attract the clips from the same video and repel the clips from different videos in the embedding space.

DeCLUTR is an approach for learning universal sentence embeddings that utilizes a self-supervised objective that does not require labelled training data. The objective learns universal sentence embeddings by training an encoder to minimize the distance between the embeddings of textual segments randomly sampled from nearby in the same document.

In this work, we present a conceptually simple and effective method to train a strong bilingual multimodal representation model. Starting from the pretrained multimodal representation model CLIP released by OpenAI, we switched its text encoder with a pretrained multilingual text encoder XLM-R, and aligned both languages and image representations by a two-stage training schema consisting of teacher learning and contrastive learning. We validate our method through evaluations of a wide range of tasks. We set new state-of-the-art performances on a bunch of tasks including ImageNet-CN, Flicker30k- CN, and COCO-CN. Further, we obtain very close performances with CLIP on almost all tasks, suggesting that one can simply alter the text encoder in CLIP for extended capabilities such as multilingual understanding. Our models and code are available at https://github.com/FlagAI-Open/FlagAI.

learn to explore the sample relationships via transformer networks

CV-MIM, or Contrastive Cross-View Mutual Information Maximization, is a representation learning method to disentangle pose-dependent as well as view-dependent factors from 2D human poses. The method trains a network using cross-view mutual information maximization, which maximizes mutual information of the same pose performed from different viewpoints in a contrastive learning manner. It further utilizes two regularization terms to ensure disentanglement and smoothness of the learned representations.

Contrastive learning has achieved remarkable success in representation learning via self-supervision in unsupervised settings. However, effectively adapting contrastive learning to supervised learning tasks remains as a challenge in practice. In this work, we introduce a dual contrastive learning (DualCL) framework that simultaneously learns the features of input samples and the parameters of classifiers in the same space. Specifically, DualCL regards the parameters of the classifiers as augmented samples associating to different labels and then exploits the contrastive learning between the input samples and the augmented samples. Empirical studies on five benchmark text classification datasets and their low-resource version demonstrate the improvement in classification accuracy and confirm the capability of learning discriminative representations of DualCL.

FashionCLIP is a fine-tuned CLIP model on fashion data (more than 800K pairs). It is the first foundation model for Fashion.

In the OneR method, model input can be one of image, text or image+text, and CMC objective is combined with the traditional image-text contrastive (ITC) loss. Masked modeling is also carried out for all three input types (i.e., image, text and multi-modal). This framework employs no modality-specific architectural component except for the initial token embedding layer, making our model generic and modality-agnostic with minimal inductive bias.

Viewmaker Network is a type of generative model that learns to produce input-dependent views for contrastive learning. This network is trained jointly with an encoder network. The viewmaker network is trained adversarially to create views which increase the contrastive loss of the encoder network. Rather than directly outputting views for an image, the viewmaker instead outputs a stochastic perturbation that is added to the input. This perturbation is projected onto an $\mathcal{l}\_{p}$ sphere, controlling the effective strength of the view, similar to methods in adversarial robustness. This constrained adversarial training method enables the model to reduce the mutual information between different views while preserving useful input features for the encoder to learn from. Specifically, the encoder and viewmaker are optimized in alternating steps to minimize and maximize $\mathcal{L}$, respectively. An image-to-image neural network is used as the viewmaker network, with an architecture adapted from work on style transfer. This network ingests the input image and outputs a perturbation that is constrained to an $\ell_{1}$ sphere. The sphere's radius is determined by the volume of the input tensor times a hyperparameter $\epsilon$, the distortion budget, which determines the strength of the applied perturbation. This perturbation is added to the input image and optionally clamped in the case of images to ensure all pixels are in $[0,1]$.

Contrastive Predictive Coding v2 (CPC v2) is a self-supervised learning approach that builds upon the original CPC with several improvements. These improvements include: - Model capacity - The third residual stack of ResNet-101 (originally containing 23 blocks, 1024-dimensional feature maps, and 256-dimensional bottleneck layers), is converted to use 46 blocks, with 4096-dimensional feature maps and 512-dimensional bottleneck layers: ResNet-161. - Layer Normalization - The authors find CPC with batch normalization harms downstream performance. They hypothesize this is due to batch normalization allowing large models to find a trivial solution to CPC: it introduces a dependency between patches (through the batch statistics) that can be exploited to bypass the constraints on the receptive field. They replace batch normalization with layer normalization. - Predicting lengths and directions - patches are predicted with contexts from both directions rather than just spatially underneath. - Patch-based Augmentation - Utilising "color dropping" which randomly drops two of the three color channels in each patch, as well as random horizontal flips. Consistent with prior results, this new architecture delivers better performance regardless of

InfoNCE variants demonstrate direct and indirect coupling between the alignment and uniformity terms thus hurting optimisation. The Decoupled Hyperspherical Energy Loss (DHEL) is an NT-Xent variant that completly decouples alignment from uniformity by discarding the corresponding terms from the denominator. In this way optimisation is more efficient and robust to hyper parameter changes.

NeuroTactic is a model for theorem proving which leverages graph neural networks to represent the theorem and premises, and applies graph contrastive learning for pre-training. Specifically, premise selection is designed as a pretext task for the graph contrastive learning approach. The learned representations are then used for the downstream task, tactic prediction

OA-Loss reduces the domain gap between the original and augmented images. OA-Loss enables the model to learn semantic relations of foreground and background instances from multi-domain.