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Title: Fix your classifier: the marginal value of training the last weight layer

Paper content:
Neural networks are commonly used as models for classification for a wide variety of tasks.
Typically, a learned affine transformation is placed at the end of such models, yielding a per-class value used for classification.
This classifier can have a vast number of parameters, which grows linearly with the number of possible classes, thus requiring increasingly more resources.
In this work we argue that this classifier can be fixed, up to a global scale constant, with little or no loss of accuracy for most tasks, allowing memory and computational benefits.
Moreover, we show that by initializing the classifier with a Hadamard matrix we can speed up inference as well.
We discuss the implications for current understanding of neural network models.
Deep neural network have become a widely used model for machine learning, achieving state-ofthe-art results on many tasks.
The most common task these models are used for is to perform classification, as in the case of convolutional neural networks (CNNs) used to classify images to a semantic category.
CNN models are currently considered the standard for visual tasks, allowing far better accuracy than preceding approaches BID21 BID42 .Training
NN models and using them for inference requires large amounts of memory and computational resources, thus, extensive amount of research has been done lately to reduce the size of networks. BID6 used
weight sharing and specification, BID28 used mixed precision to reduce the size of the neural networks by half. BID44 and
BID19 used low rank approximations to speed up NNs. BID17 , BID24
and BID51 , used a more aggressive approach, in which weights, activations and gradients were quantized to further reduce computation during training. Although aggressive
quantization benefits from smaller model size, the extreme compression rate comes with a loss of accuracy.Past work noted the fact that predefined BID31 and random BID15 projections can be used together with a learned affine transformation to achieve competitive results on several tasks. In this study suggest
the reversed proposal -that common NN models used can learn useful representation even without modifying the final output layer, which often holds a large number of parameters that grows linearly with number of classes.
In this work we suggested removing the parameters from the classification layer used in deep neural networks.
We showed empirical results suggesting that keeping the classifier fixed cause little or no decline in classification performance for common balanced datasets such as Cifar and Imagenet, while allowing a noticeable reduction in trainable parameters.
We argue that fixing the last layer can reduce the computational complexity for training as well as the communication cost in distributed learning.
Furthermore, using a Hadamard matrix as classifier might lead to some computational benefits when properly implemented, and save memory otherwise spent on large amount of transformation coefficients.
As datasets tend to become more complex by time (e.g., Cifar100, ImageNet1K, ImageNet22k, JFT-300M, and language modeling) we believe that resource hungry affine transformation should remain fixed during training, at least partially.We also found that new efficient methods to create pre-defined word embeddings should be explored, as they require huge amount of parameters that can possibly be avoided when learning a new task.
Based on these findings, we recommend future research to focus on representations learned by the non-linear part of neural networks -up to the final classifier, as it seems to be highly redundant.

assistant: text: You can fix the classifier in neural networks without losing accuracy