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The Lottery Ticket Hypothesis
• Last Updated : 10 Jun, 2020

The Lottery Ticket Hypothesis has been presented in the form of a research paper at ICLR 2019 by MIT-IBM Watson AI Lab. This paper has been awarded the Best Paper Award in ICLR 2019.

Background: Network Pruning
Pruning basically means reducing the extent of a neural network by removing superfluous and unwanted parts. Network Pruning is a commonly used practise to reduce the size, storage and computational space occupied by a neural network. Like – Fitting an entire neural network in your phone. The idea of Network Pruning was originated in the 1990s which was later popularized in 2015.

How do you “prune” a neural network?
We can summarize the process of pruning into 4 major steps:

1. Train the Network
2. Remove superflous structures
3. Fine-tune the network
4. Optionally : Repeat the Step 2 and 3 iteratively

But, before we further move ahead, you must know :

• Usually, pruning is done after a neural network is trained on data.
• The superfluous structures can be Weights, Neurons, Filters, Channels . However, here we consider “sparse pruning” which means pruning “weights”.
• A heuristic is needed to define whether a structure is superfluous or not. These heuristics are Magnitudes, Gradients, or Activations. Here, we chose magnitudes. We prune the weights with the lowest magnitudes.
• By removing parts out of neural network, we somewhat have damaged the activation function. Hence, we train the model a bit more. This is known as fine-tuning.
If the steps are correctly followed, we can compress the parameters of neural networks like LeNet-300-100 and AlexNet by a compression rate of 9x to 12x without losing any accuracy.

Can’t we randomly initialize a pruned network and train to convergence?
Many researchers have pondered over this conclusion. However, all of them came with the same answer – No.
It turns out that Training a pruned model from scratch performs worse than retraining a pruned model, which may indicate the difficulty of training a network with small capacity .

However, this is no longer the case. The research conducted by MIT-IBM shows that we can indeed train pruned networks from scratch. Also, there exist no need for networks to be overparameterized to learn. Weights pruned after training could have been pruned before training however, you need to use the same intializations.

How to train pruned networks ?
1. Randomly intialize the full network
2. Train it and prune superflous structure
3. Reset each remaining weight to its value after Step 1.

This basically suggests that “There exists a subnetwork that exists inside a randomly-initialized deep neural network which when trained in isolation can match or even outperform the accuracy of the original network.

• A fully-connected neural network like MNIST having more than 600K parameters supposedly is reduced to a subnet of 21K parameters having the same accuracy as the original network
• Retention of the the original features – Dropout, weight decay, batchnorm, resnet, your favourite optimizer etc.

Further Scope of Research

• Subnetworks are found retroactively
• Finding subnetworks is very expensive
• Small, vision networks and tasks

Link to the research paper: The lottery ticket hypothesis: Finding sparse, trainable neural networks

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