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Title: ROBUST DISCRIMINATIVE REPRESENTATION LEARNING VIA GRADIENT RESCALING: AN EMPHASIS REGULARISATION PERSPECTIVE

Paper content:
It is fundamental and challenging to train robust and accurate Deep Neural Networks (DNNs) when semantically abnormal examples exist.
Although great progress has been made, there is still one crucial research question which is not thoroughly explored yet: What training examples should be focused and how much more should they be emphasised to achieve robust learning?
In this work, we study this question and propose gradient rescaling (GR) to solve it.
GR modifies the magnitude of logit vector’s gradient to emphasise on relatively easier training data points when noise becomes more severe, which functions as explicit emphasis regularisation to improve the generalisation performance of DNNs.
Apart from regularisation, we connect GR to examples weighting and designing robust loss functions.
We empirically demonstrate that GR is highly anomaly-robust and outperforms the state-of-the-art by a large margin, e.g., increasing 7% on CIFAR100 with 40% noisy labels.
It is also significantly superior to standard regularisers in both clean and abnormal settings.
Furthermore, we present comprehensive ablation studies to explore the behaviours of GR under different cases, which is informative for applying GR in real-world scenarios.
DNNs have been successfully applied in diverse applications (Socher et al., 2011; Krizhevsky et al., 2012; LeCun et al., 2015) .
However, their success is heavily reliant on the quality of training data, especially accurate semantic labels for learning supervision.
Unfortunately, on the one hand, maintaining the quality of semantic labels as the scale of training data increases is expensive and almost impossible when the scale becomes excessively large.
On the other hand, it has been demonstrated that DNNs are capable of memorising the whole training data even when all training labels are random (Zhang et al., 2017) .
Therefore, DNNs struggle to discern meaningful data patterns and ignore semantically abnormal examples 1 simultaneously (Krueger et al., 2017; Arpit et al., 2017) .
Consequently, it becomes an inevitable demand for DNNs to hold robustness when training data contains anomalies (Larsen et al., 1998; Natarajan et al., 2013; Sukhbaatar & Fergus, 2014; Xiao et al., 2015; Patrini et al., 2017; Vahdat, 2017; Veit et al., 2017; Li et al., 2017) .
Recently, great progress has been made towards robustness against anomalies when training DNNs (Krueger et al., 2017) .
There are three appealing perspectives in terms of their simplicity and effectiveness:
1) Examples weighting.
For example, knowledge distilling from auxiliary models is popular for heuristically designing weighting schemes.
However, it is challenging to select and train reliable auxiliary models in practice (Li et al., 2017; Malach & Shalev-Shwartz, 2017; Jiang et al., 2018; Ren et al., 2018; Han et al., 2018b) .
2) Robust loss functions (Van Rooyen et al., 2015; Ghosh et al., 2017; Zhang & Sabuncu, 2018; Wang et al., 2019b) ; 3) Explicit regularisation techniques (Arpit et al., 2017; .
Although designing robust losses or explicit regularisation is easier and more flexible in practice, the performance is not the optimal yet.
1 One training example is composed of an input and its corresponding label.
A semantically abnormal example means the input is semantically unrelated to its label, which may come from corrupted input or label.
For example, in Figure 3 in the supplementary material:
1) Out-of-distribution anomalies: An image may contain only background or an object which does not belong to any training class;
2) In-distribution anomalies: An image of class a may be annotated to class b or an image may contain more than one semantic object.
Regarding examples weighting, there is a core research question which is not well answered yet:
What training examples should be focused on and how large the emphasis spread should be?
In this work, we present a thorough study of this practical question under different settings.
For better analysis, we propose two basic and necessary concepts: emphasis focus and spread with explicit definition in Sec. 3.2.
They are conceptually introduced as follows:
Emphasis focus.
It is a common practice to focus on harder instances when training DNNs (Shrivastava et al., 2016; Lin et al., 2017) .
When a dataset is clean, it achieves faster convergence and better performance to emphasise on harder examples because they own larger gradient magnitude, which means more information and a larger update step for model's parameters.
However, when severe noise exists, as demonstrated in (Krueger et al., 2017; Arpit et al., 2017) , DNNs learn simple meaningful patterns first before memorising abnormal ones.
In other words, anomalies are harder to fit and own larger gradient magnitude in the later stage.
Consequently, if we use the default sample weighting in categorical cross entropy (CCE) where harder samples obtain higher weights, anomalies tend to be fitted well especially when a network has large enough capacity.
That is why we need to move the emphasis focus towards relatively easier ones, which serves as emphasis regularisation.
Emphasis spread.
We term the weighting variance of training examples emphasis spread.
The key concept is that we should not treat all examples equally, neither should we let only a few be emphasised and contribute to the training.
Therefore, when emphasis focus changes, the emphasis spread should be adjusted accordingly.
We integrate emphasis focus and spread into a unified example weighting framework.
Emphasis focus defines what training examples own higher weights while emphasis spread indicates how large variance over their weights.
Specifically, we propose gradient rescaling (GR), which modifies the magnitude of logit vector's gradient.
The logit vector is the output of the last fully connected (FC) layer of a network.
We remark that we do not design the weighting scheme heuristically from scratch.
Instead, it is naturally motivated by the gradient analysis of several loss functions.
Interestingly, GR can be naturally connected to examples weighting, robust losses, explicit regularisation:
1) The gradient magnitude of logit vector can be regarded as weight assignment that is built-in in loss functions (Gopal, 2016; Alain et al., 2016; Zhang et al., 2018b) .
Therefore, rescaling the gradient magnitude equals to adjusting the weights of examples;
2) A specific loss function owns a fixed gradient derivation.
Adjusting the gradient can be treated as a more direct and flexible way of modifying optimisation objectives;
3) Instead of focusing on harder examples 2 by default, we can adjust emphasis focus to relative easier ones when noise is severe.
GR serves as emphasis regularisation and is different from standard regularisers, e.g., L2 weight decay constraints on weight parameters and Dropout samples neural units randomly (Srivastava et al., 2014) ; GR is simple yet effective.
We demonstrate its effectiveness on diverse computer vision tasks using different net architectures:
1) Image classification with clean training data;
2) Image classification with synthetic symmetric label noise, which is more challenging than asymmetric noise evaluated by (Vahdat, 2017; ; 3) Image classification with real-world unknown anomalies, which may contain open-set noise , e.g., images with only background, or outliers, etc.
;
4) Video person re-identification, a video retrieval task containing diverse anomalies.
Beyond, we show that GR is notably better than other standard regularisers, e.g., L2 weight decay and dropout.
Besides, to comprehensively understand GR's behaviours, we present extensive ablation studies.
Main contribution.
Intuitively and principally, we claim that two basic factors, emphasis focus and spread, should be babysat simultaneously when it comes to examples weighting.
To the best of our knowledge, we are the first to thoroughly study and analyse them together in a unified framework.
In this work, we present three main contributions:
1) We analyse and answer a core research question: What training examples should be focused on and how large the emphasis spread should be?
2) We uncover and analyse that two basic factors, emphasis focus and spread, should be babysat simultaneously when it comes to examples weighting.
Consequently, we propose a simple yet effective gradient rescaling framework serving as emphasis regularisation.
3) Extensive experiments on different tasks using different network architectures are reported for better understanding and demonstration of GR's effectiveness, which are also valuable for applying GR in practice.
(Zheng et al., 2016) .
Out-of-distribution anomalies:
1) The first image in the 3rd row contains only background and no semantic information at all.

assistant: text: ROBUST DISCRIMINATIVE REPRESENTATION LEARNING VIA GRADIENT RESCALING: AN EMPHASIS REGULARISATION PERSPECTIVE