Coursera deep learning specialization notes and code [Specialization Page]

Notes are taken in the format of QA.

Move to Note.md since some formula images are not rendered 😢

• Course 1: Neural Networks and Deep Learning
  • Week 3: Shallow Neural Networks
  • Week 4: Deep Neural Networks
    • Programming: Building your Deep Neural Network: Step by Step (Code)
    • Programming: Deep Neural Network - Application
• Course 2: Improving Deep Neural Networks: Hyperparameter Tuning, Regularization and Optimization
  • Week 1: Practical Aspects of Deep Learning
  • Week 2: Optimization Algorithms
  • Week 3: Hyperparameter Tuning, Batch Normalization and Programming Frameworks
• Course 3: Structuring Machine Learning Projects
  • Week 1: ML Strategy 1
  • Week 2: ML Strategy 2
• Course 4: Convolutional Neural Networks
  • Week 1: Foundations of Convolutional Neural Networks
    • Programming: Convolutional Model, Step by Step
    • Programming: Convolution Model Application (Keras)(Pytorch)
  • Week 2: Deep Convolutional Models: Case Studies
    • Programming: Residual Networks
    • Programming: Transfer Learning with MobileNet
  • Week 3: Detection Algorithms
    • Programming: Car Detection with YOLO
    • Programming: Image Segmentation with U-Net
  • Week 4: Face Recognition
    • Programming: Face Recognition
    • Programming: Art Generation with Neural Style Transfer
• Course 5: Sequence Model
  • Week 1: Recurrent Neural Network
    • Programming: Building your Recurrent Neural Network - Step by Step
    • Programming: Dinosaur Island-Character-Level Language Modeling
    • Programming: Jazz Improvisation with LSTM
  • Week 2: Natural Language Processing & Word Embeddings
    • Programming: Operations on Word Vectors - Debiasing
    • Programming: Emojify
  • Week 3: Sequence Models & Attention Mechanism
    • Programming: Neural Machine Translation
    • Programming: Trigger Word Detection
  • Week 4: Transformer Network
    • Programming: Transformers Architecture with TensorFlow (Keras)
    • Lab: Transformer Pre-processing
    • Lab: Transformer Network Application: Named-Entity Recognition
    • Lab: Transformer Network Application: Question Answering
    • Lab: Transformers using Trax Library
• Further Exploration
  • Vision Transformer

Coursera Syllabus

YouTube Playlist

Week 1 is the overview of the course and specialization.

What is the difference between structure and unstructured data?

What are the dimensions of input matrix and weights?

Param	Description
	number of observations
	number of features (input data)
	number of layers. : input layer
	number of units (features) at layer .

Matrix	Shape

To better memberize

:

num of row: number of units of the next layer
num of col: number of units of the current layer

and :

num of row: number of units of the next layer
num of col: number of observations

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What are the pros and cons of activation functions?

Activation Function	Output	Characteristic
Sigmoid	(0, 1)	good for output layer of binary classification
Tanh	(-1, 1)	center data, mean of data is close to 0, makes learning for next layer a bit easier
ReLU	(0, )	derivative of slope is 1 when z > 0, is 0 when z < 0
Leasky ReLU	(-, )

Why non-linear activation functions?

If we use linear activation functions, no matter how many layers you have, the NN is just computing a linear function.

Why do we usually initialize as small random values?

large W -> large Z (Z = WX + b) -> end up at the flat parts of Sigmoid function -> gradient will be small -> gradient descent will be slow -> learning will be slow

If you're not using Sigmoid or Tanh activation functions, it is less of an issue. But note if you're doing a binary classification, the output layer will be a Sigmoid function.

What distribution should we draw from?

Normal distirbution.

In Python we should use np.random.randn (normal distribution) instead of np.random.rand (uniform distribution).

Logistic regression’s weights w should be initialized randomly rather than to all zeros, because if you initialize to all zeros, then logistic regression will fail to learn a useful decision boundary because it will fail to “break symmetry”, True/False?

False. Logistic Regression doesn't have a hidden layer. If you initialize the weights to zeros, the first example x fed in the logistic regression will output zero but the derivatives of the Logistic Regression depend on the input x (because there's no hidden layer) which is not zero.

So at the second iteration, the weights values follow x's distribution and are different from each other if x is not a constant vector.

But in deep learning we should randomly initialize either or to "break symmetry". If both and values zero, will be 0 because tanh(0)=0. Using non-zero initialization but making them all the same does not work either. Though we can learn new values, but the values we get are symmetric, means it's the same as a network with a single neuron.

Reference: Symmetry Breaking versus Zero Initialization

A = np.random.randn(4,3); B = np.sum(A, axis = 1, keepdims = True). What will be B.shape?

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What is the relationship between # of hidden units and # of layers?

Informally: for equal performance shallower networks require exponentially more hidden units to compute.

What is the intuition about deep representation?

Intuitively, deeper layers compute more complex things such as eyes instead of edges.

Vectorization allows you to compute forward propagation in an LL-layer neural network without an explicit for-loop (or any other explicit iterative loop) over the layers l=1, 2, …,L. True/False?

False. Forward propagation propagates the input through the layers, although for shallow networks we may just write all the lines. In a deeper network, we cannot avoid a for loop iterating over the layers.

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Coursera Syllabus

YouTube Playlist

What are the differences when creating train, dev, test sets in traditional ML and DL?

In traditional ML, train/dev/test split may be 60% / 20% / 20%.

In DL, since the data is large, train/dev/test may be 99.5% / 0.4% / 0.1%

Side note: not having a test set might be okay.

What should we do if the variance or bias is high?

Why regularization reduces overfitting?

If lambda is large, weights will be small or close to zero because gradient descent minimizes the cost function.

small weights -> decrease impacts of some hidden units -> simpler network -> not overfit

What are the differences between L1 and L2 regularization?

Regularization	Penalize		Feature selection
L1	sum of absolute values of the weights	sparse	Yes
L2	sum of squares of the weights	non-sparse	No

What is dropout regularization? Why does it work?

Dropout regularization randomly switch off some hidden units so they do not learn anything and the NN will be simpler

What should we pay attention to when implementing dropout during train / test time?

D1 = np.rand(a, b)
D1 = (D1 < keep_prob).astype(int)
A1 = A1 * D1 
A1 = A1 / keep_prob

Note the devriatives during backward also need to scale

dA1 = dA1 * D1
dA1 = dA1 / keep_prob

What is weight decay?

A regularization technique (such as L2 regularization) that results in gradient descent shrinking the weights on every iteration.

Why do we normalize the inputs x?

It makes the cost function easier and faster to optimize

What is vanishing / exploding gradient ?

Derivatives of each layer are multiplied layer by layer (inputs times gradient). If we have a sigmoid or tanh activation function, derivates are always a fraction.

During backpropagate, fractions are multiplying for many times, the gradient decreases expotentially and the weights of the initial layer will be very small, which makes it hard to learn.

How to deal with vanishing gradient?

A partial solution: force the variance of to be constant and smaller. A recommended value is but it depends on the activation function.

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What are the differences between batch , mini-batch, and stochatic gradient descent?

How to choose mini-batch size?

if m <= 2000: 
    use batch gd
else: 
    typical size: 2_4, 2_5, 2_6... 

It depends on the context, we should test with different sizes 

Formula of bias correction in exponentially weighted averages?

What is momentum?

Momentum is a method to dampen down the changes in gradients and accelerate gradients vectors in the right direction using exponentially weighted averages.

Which of these is NOT a good learning rate decay scheme? Here, t is the epoch number.

What is the process of parameter update in Adam?

Why batch normalization?

Normalization can make training faster and hyperparameters more robust.

Values of each hidden layer are changing all the time because of changes in and , suffering from the problem of covariance shift. Batch normalization guarantees the mean and variance of features of each layer (e.g., , ) keep the same no matter how actual values of each node changes. It allows each layer to learn by itself (more independently than no batch normalization), and speed up learning.

Mean and variance are governed by two learnable parameters and . Adding and is because we don't want all the layers have the same mean and variance (mean = 0, variance = 1).

Batch normalization fomula?

If searching among a large number of hyperparameters, should you try values in a gird or by random? Why

Random.

Grid method is okay if # of hyperparameter is small. In DL, it is difficult to know in advance which hyperparameter is more important. Random method allow us to try more distinct values that are potentially important.

What the hyperparamers and their default values?

Hyperparameter	common value
learning rate
momentum	around 0.9
mini-batch size
# of hidden units	-
learning rate decay
# of layers	-
batch normalization , ,	0.9, 0.99,

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What are the types of metrics?

Optimizing metric: the metric you want as good as possible, e.g., accuracy

Satisficing metric: as long as it reaches a threshold, e.g., run time, memory

How should we make decisions on train/dev set error ?

We should always have Bayes error to estimate avoidable bias. Human-level error is often seen as a proxy of Bayes eror.

A learning algorithm’s performance can be better human-level performance but it can never be better than Bayes error. human-level performance

We should not add data from a different distribution to the train set. True / False?

False.

Sometimes we'll need to train the model on the data that is available, and its distribution may not be the same as the data that will occur in production. Also, adding training data that differs from the dev set may still help the model improve performance on the dev set. What matters is that the dev and test set have the same distribution.

We should not add data from a different distribution to the test set. True / False?

True.

This would cause the dev and test set distributions to become different.

What should you do if another metric (e.g., false negative rate) should be taken into account?

Rethink the appropriate metric for this task, and ask your team to tune to the new metric.

A softmax activation would be a good choice for the output layer if this is a multi-task learning problem. True/False?

False. Softmax would be a good choice if one and only one of the possibilities (stop sign, speed bump, pedestrian crossing, green light and red light) was present in each image.

Should you correct mislabeled data in train and test set after you did so in dev set?

You should correct mislabeled data in test set because test and dev set should come from the same distribution.

You do not necessarily need to fix the mislabeled data in the train set because it's okay for the train set distribution to differ from the dev and test sets.

Let's say you have 100,000 images taken by cars' front camera (you care about) and 900,000 images from the internet. How should you split train/test set?

One example:

Train set: 900,000 images from the internet + 80,000 images from car’s front-facing camera.

Dev / Test set: The 20,000 remaining front-camera images in each set.

As seen in lecture, it is important that your dev and test set have the closest possible distribution to “real”-data. It is also important for the training set to contain enough “real”-data to avoid having a data-mismatch problem.

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What are the problems of convolution?

1. Each time you apply a convolution operator, the image shrinks.
2. Pixels on the corner or edge will be used much less than those in the middle.

Notations and dimensions of input matrix and parameters

Param	Description
	filter size
	padding
	stride
	number of filters

Metric	Dimension
Filter
Activations
Weights
bias

What is valid and same convolutions?

Valid: no padding

Same: Pad so that output size is the same as the input size

How to calculate the dimension of next conv layer?

Input is a 300 by 300 color (RGB) image, and you use a convolutional layer with 100 filters that are each 5x5. How many parameters does this hidden layer have (including the bias parameters)?

What are the benefits of CNN?

1. It allows a feature detector to be used in multiple locations throughout the whole input volume.

2. Convolutional layers provide sparsity of connections.

3. Invariance

What does “sparsity of connections” mean?

Each activation in the next layer depends on only a small number of activations from the previous layer.

Each activation of the output volume is computed by multiplying the parameters from only one filter with a volumic slice of the input volume and then summing all these together.

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LeNet - 5

LeCun, Y., Bottou, L., Bengio, Y., & Haffner, P. (1998). Gradient-based learning applied to document recognition. Proceedings of the IEEE, 86(11), 2278-2324.

hard to read

AlexNet

Krizhevsky, A., Sutskever, I., & Hinton, G. E. (2012). Imagenet classification with deep convolutional neural networks. Advances in neural information processing systems, 25.

easy to read

VGG - 16

Simonyan, K., & Zisserman, A. (2014). Very deep convolutional networks for large-scale image recognition. arXiv preprint arXiv:1409.1556.

ResNet

He, K., Zhang, X., Ren, S., & Sun, J. (2016). Deep residual learning for image recognition. In Proceedings of the IEEE conference on computer vision and pattern recognition (pp. 770-778).

What does skip-connection do?

Skip-connections make it easy for the network to learn an identity mapping between the input and the output within the ResNet block.

Why does ResNet work?

It helps with gradient vanishing and exploding problems and allows people to train deep neural networks without loss in performance.

"The skip connections in ResNet solve the problem of vanishing gradient in deep neural networks by allowing this alternate shortcut path for the gradient to flow through. The other way that these connections help is by allowing the model to learn the identity functions which ensures that the higher layer will perform at least as good as the lower layer, and not worse. "

-- Introduction to Resnet or Residual Network

Inceptionm Network

Szegedy, C., Liu, W., Jia, Y., Sermanet, P., Reed, S., Anguelov, D., ... & Rabinovich, A. (2015). Going deeper with convolutions. In Proceedings of the IEEE conference on computer vision and pattern recognition (pp. 1-9).

MobileNet

Howard, A. G., Zhu, M., Chen, B., Kalenichenko, D., Wang, W., Weyand, T., ... & Adam, H. (2017). Mobilenets: Efficient convolutional neural networks for mobile vision applications. arXiv preprint arXiv:1704.04861.

Suppose that in a MobileNet v2 Bottleneck block we have an input volume. We use 30 filters for the expansion. In the depthwise convolutions we use filters, and 20 filters for the projection.

How many parameters are used in the complete block, suppose we don't use bias?

Expansion filter: 5 * 30 = 150

Depthwise: 3 * 3 * 30 = 270

Pointwise: 30 * 20 = 600

Total = 150 + 270 + 600 = 1020

EfficientNet

Tan, M., & Le, Q. (2019, May). Efficientnet: Rethinking model scaling for convolutional neural networks. In International conference on machine learning (pp. 6105-6114). PMLR.

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YOLO

Redmon, J., Divvala, S., Girshick, R., & Farhadi, A. (2016). You only look once: Unified, real-time object detection. In Proceedings of the IEEE conference on computer vision and pattern recognition (pp. 779-788).

Github: YAD2K: Yet Another Darknet 2 Keras

What are advantages of YOLO algorithm?

• It can ouput more accurate bounding box
• It's one single convolutional computation where you use one conv net with lots of shared computation between all the computation needed for all the cells. Therefore it's a very efficient algorithm.

How to evaluate object localization?

How does non-max suppression work?

While BoundingBoxes:

• Pick the box with the highest score (), add it to ouput list
• Calculate the overlap IoU of this box and all the other boxes. Discard boxes that overlap significantly (iou >= iou_threshold).
• Repeat the steps above until there are no more boxes with a lower score than the currently selected box.

What is the dimension of one grid in YOLO? Suppose there are classes and anchors

How does Transposed Convolution work?

Transposed Convolutions are used to upsample the input feature map to a desired output feature map using some learnable parameters.

- pick the top left corner element of input, multiply it with every element in the kernel
- put the result (the same size with kernel) on the top left corner of output matrix
- pick the second element of input, multiple it with every element in the kernel
- put the result in the output matrix based on stride
- repeat the steps above 
- if there is overlap of results, add the elements 
- ignore elements in the padding

Read more:

Make your own neural network | Calculating the Output Size of Convolutions and Transpose Convolutions

Towards Data Science | Transposed Convolution Demystified

U-net

Ronneberger, O., Fischer, P., & Brox, T. (2015, October). U-net: Convolutional networks for biomedical image segmentation. In International Conference on Medical image computing and computer-assisted intervention (pp. 234-241). Springer, Cham.

What is the dimension of U-Net archiecture output?

where is the number of classes

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What is the differences between face verification and face recognition?

Siamese Network

Taigman, Y., Yang, M., Ranzato, M. A., & Wolf, L. (2014). Deepface: Closing the gap to human-level performance in face verification. In Proceedings of the IEEE conference on computer vision and pattern recognition (pp. 1701-1708).

Triplet Loss

Schroff, F., Kalenichenko, D., & Philbin, J. (2015). Facenet: A unified embedding for face recognition and clustering. In Proceedings of the IEEE conference on computer vision and pattern recognition (pp. 815-823).

Neural Style Transfer

Zeiler, M. D., & Fergus, R. (2014, September). Visualizing and understanding convolutional networks. In European conference on computer vision (pp. 818-833). Springer, Cham.

Gatys, L. A., Ecker, A. S., & Bethge, M. (2015). A neural algorithm of artistic style. arXiv preprint arXiv:1508.06576.

What is the cost function of style transfer?

: generated image.

: content image.

: style image.

: gram matrix

Content cost. For each layer:

Content cost. For all the entries

Style cost. For each layer:

Style cost. For all entries:

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Why not use standard network on sequence data?

• Inputs and outputs can be different in length.
• Standard network doesn't share features across positions of text. E.g., Harry at position 0 is a name, is other Harry at other positions also a name?

Notations

Param	Description
	the t th element in the training sequence i
	the length of training sequence i
	the t th element in the output sequence i
	the length of output sequence i

What is the formula of forward propagation?

Here , the second index means will be multiplied by some x-like quantity, to compute some a-like quantity.

List some examples of RNN architectures

• Many to one: sentiment classiciation
• One to many: music generation. Input: genre / first note; output: a sequence of notes
• Many to many (different length): machine translation.

Andrej Karpathy: The Unreasonable Effectiveness of Recurrent Neural Networks

Why RNN has vanishing gradients problems?

An ouput is mainly influenced by values close to its position.
It's difficult for an output to be strongly influenced by an input that is very early in the sequence. Because it's difficult to backpropagate all the way to the beginning of the sequence.

E.g., The cat, which already ate, was full.

How to deal with exploding gradients?

Apply gradients clipping. Re-scale some gradient vectors when it's bigger than some threshold.

Where and how do you apply clipping?

forward pass -> cost computation -> backward pass -> CLIPPING -> parameter update

np.clip(gradient, -maxValue, maxValue, out = gradient)

def optimize(X, Y, a_prev, parameters, learning_rate): 

    loss, cache = rnn_forward(X, Y, a_prev, parameters)
    
    gradients, a = rnn_backward(X, Y, parameters, cache)
    
    gradients = clip(gradients, 5)
    
    parameters = update_parameters(parameters, gradients, learning_rate)
        
    return loss, gradients, a[len(X)-1]

How to deal with vanishing gradients?

Vanichsing gradient problems are harder to detect and solve. Gated Recurrent Unit (GRU) is an effective solution.

What is the formula of full Gated Recurrent Unit (GRU)?

: memory cell

: output activation. They're the same here, but different in LSTM

: how relevant is to

: update gate, (0,1). Either close to 0 or 1

Cho, K., Van Merriënboer, B., Bahdanau, D., & Bengio, Y. (2014). On the properties of neural machine translation: Encoder-decoder approaches.

Chung, J., Gulcehre, C., Cho, K., & Bengio, Y. (2014). Empirical evaluation of gated recurrent neural networks on sequence modeling.

Hochreiter, S., & Schmidhuber, J. (1997). Long short-term memory. Neural computation, 9(8), 1735-1780.

How does LSTM differ from GRU?

Instead of having one update gate controls and , LSTM has two separate gates and (forget gate).

Update :

while in GRU, it's

What are the disadvantages of Bidirectional RNN?

You do need an entire sequence of date before making predictions anywhere (cannot use in real-time application).

True/False: In RNN, step t uses the probabilities output by the RNN to pick the highest probability word for that time-step. Then it passes the ground-truth word from the training set to the next time-step.

No, the probabilities output by the RNN are not used to pick the highest probability word and the ground-truth word from the training set is not the input to the next time-step.

You find your weights and activations are all taking on the value of NaN (“Not a Number”), what problem may cause it?

Gradient exploding. It happens when large error gradients accumulate and result in very large updates to the NN model weights during training. These weights can become too large and cause an overflow, identified as NaN.

Alice proposes to simplify the GRU by always removing the . I.e., setting = 0. Betty proposes to simplify the GRU by removing the . I. e., setting = 1 always. Which of these models is more likely to work without vanishing gradient problems even when trained on very long input sequences?

• Alice's model (removing ). Because if for a timestep, the gradient can propagate back through that timestep without much decay.
• Alice's model (removing ). Because if for a timestep, the gradient can propagate back through that timestep without much decay.
• Betty's model (removing ). Because if for a timestep, the gradient can propagate back through that timestep without much decay.
• Betty's model (removing ). Because if for a timestep, the gradient can propagate back through that timestep without much decay.

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What is the downside of skip-gram model?

The Softmax objective is expensive to compute because it needs to sum over the entire vocabulary.

What are differences of problem objectives in the skip-gram model and negative sampling?

Skip-gram: given a context, predict the probability of different target word

Negative sampling: given a pair of words, is it a context-target pair? Is it a positive or negative sample?

Why negative sampling's computation cost is lower?

It converts a N softmax problem to a N binary classification problem. In each iteration, only train K words. K = 5 to 20 in small vocabulary, K = 2 to 5 in large vobabulary.

What is the learning objective of GloVe?

= # of times appears in the context of

depending on the definition of "context", and may be symmetric.

Debiasing word embeddings

Bolukbasi, T., Chang, K. W., Zou, J. Y., Saligrama, V., & Kalai, A. T. (2016). Man is to computer programmer as woman is to homemaker? debiasing word embeddings. Advances in neural information processing systems, 29.

What are the steps in the debiasing word embeddings paper?

• Step 1: Identify gender subspace by SVD.
• Step 2a: Hard de-biasing (neutralize and equalize).
• Step 2b: Soft bias correction.

Determine gender specific words: first listed218 words from dictionary, then trained a SVM to classify 3M words in w2vNEWS, resulting in 6,449 gender-specific words.

is an embedding matrix, is a one-hot vector corresponding to word 4567. Can we call in Python to get the embedding of word 4567?

1. Input the "dummy" vector of zeros and
2. Run one step of forward pass to get and
3. Sampling the next index with the probability in . Use np.random.choice
4. Update to . Set x[idx] = 1

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Why not use greedy search?

Picking the best first word one by one does not maximize the conditional probability. The translation may be a common English sentence but not the succinct translation.

How to pick beam width?

• Large beam width: better result, slower
• Small beam width: worse result: faster

How to figure if it's RNN or beam search fails the translation task?

Compare and

> -> RNN

< -> beam search

How does sentence normalization affect beam search result?

If we carry out beam search without using sentence normalization, the algorithm will tend to output overly short translations.

What does denote in attention model?

denotes when computing the output first word, how much attention should be paid on the input second word.

is the amount of attention of should pay to

The attention model performs the same as the encoder-decoder model, no matter the sentence length. True/False?

The network learns where to “pay attention” by learning the values , which are computed using a small neural network: We can replace with as an input to this neural network because is independent of and . True/False?

• : energy variable
• : hidden state of the post-attention LSTM
• : hidden state of the pre-attention LSTM
• and are fed into a simple neural network, which learns the function to output .

What are the steps of implementing attention with Keras?

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More Resources

• The Illustrated Transformer
• YouTube | Illustrated Guide to Transformers Neural Network: A step by step explanation

How does traditional attention and self-attention in Transformer differ?

Why self-attention is used in Transformer?

What do Q, K, V denote?

What's the differences that RNN, Seq2Seq with attention, and transformer handle long sequences?

(May not be correct 😅)

|

What are criteria for a good positional encoding algorithm?

• It should output a unique encoding for each time-step (word's position in a sentence).

• Distance between any two time-steps should be consistent for all sentence length.

• The algorithm should be able to generalize to longer sentences.

What is the formula of positional encoding and what is its logic?

It records the relative positions of tokens and sums to wording embeddings. So the initial representations moves a bit towards the other tokens that are close to them.

Resources:

• Dosovitskiy, A., Beyer, L., Kolesnikov, A., Weissenborn, D., Zhai, X., Unterthiner, T., ... & Houlsby, N. (2020). An image is worth 16x16 words: Transformers for image recognition at scale. arXiv preprint arXiv:2010.11929.
• Brian Pulfer. 2022. Vision Transformers from Scratch (PyTorch): A step-by-step guide. MLearning.ai.
• Francesco Zuppichini. 2021. Vision Transformers from Scratch (PyTorch): A step-by-step guide. Towards Data Science