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*Memos:

• My post explains Recurrent Layer, LSTM, GRU and Transformer.
• My post explains activation functions in PyTorch.
• My post explains loss functions in PyTorch.
• My post explains optimizers in PyTorch.

A layer is a collection of nodes to do a specific task.

Basically, a Neural Network(NN) consists of 3 layers as shown below:

• Input Layer:

  • is the 1st layer which accepts data and pass it to a hidden layer.

• Hidden Layer:

  • is the layer between an input and output layer.
  • can be zero or more hidden layers in a neural network.

• Output layer:

  • is the last layer which holds a result.

*There is the Single Layer Neural Network or Perceptron which only has an input and output layer without hidden layer.

And, there are popular layers as shown below. *Some layers can be Neural Networks or models:

(1) Fully-connected Layer:

• connects every neuron in one layer to every neuron in the next layer.
• is also called Linear Layer, Dense Layer or Affine Layer.
• is Linear() in PyTorch. *My post explains Linear().

(2) Convolutional Layer(1982):

• can make data stand out by extracting the features from the data with filters(kernels). *Extracting the features from the data also downsamples and reduces the data to reduce computation.
• is used for Convolutional Neural Network(CNN): *Memos:
  • There are 1D CNN, 2D CNN and 3D CNN.
  • 1D CNN is for the 1D data such as the time series data such as audio, text, etc.
  • 2D CNN is for the 2D data such as a 2D image.
  • 3D CNN is for the 3D data such as video, Magnetic Resonance Imaging(MRI), Computerized Tomography(CT) Scan, etc.
• is Conv1d(), Conv2d() or Conv3d() in PyTorch: *Memos:
  • Conv1d() is for 1D data.
  • Conv2d() is for 2D data.
  • Conv3d() is for 3D data.
  • My post explains Conv1d().
  • My post explains Conv2d().
  • My post explains Conv3d().

(3) Transposed Convolutional Layer:

• can upsample data.
• is used for CNN.
• is also called Deconvolutional Layer.
• is ConvTranspose1d(), ConvTranspose2d() or ConvTranspose3d() in PyTorch: *Memos:
  • ConvTranspose1d() is for 1D data.
  • ConvTranspose2d() is for 2D data.
  • ConvTranspose3d() is for 3D data.
  • My post explains ConvTranspose1d().
  • My post explains ConvTranspose2d().
  • My post explains ConvTranspose3d().

(4) Pooling Layer:

• can downsample(reduce data) keeping features to reduce computation. *The way of downsampling data is different from Convolutional Layer.
• is used for CNN.
• Max pooling, Average pooling and Min pooling are popular. *Max, Average or Min pooling takes a maximum(brighter), average or minimum(darker) value(pixel) respectively from each filter(kernel) of an image.
• is MaxPool1d(), MaxPool2d(), MaxPool3d(), AvgPool1d(), AvgPool2d() or AvgPool3d() in PyTorch: *Memos:
  • Min pooling doesn't exist in PyTorch.
  • MaxPool1d() and AvgPool1d() are for 1D data.
  • MaxPool2d() and AvgPool2d() are for 2D data.
  • MaxPool3d() and AvgPool3d() are for 3D data.
  • My post explains MaxPool1d().
  • My post explains MaxPool2d().
  • My post explains MaxPool3d().
  • My post explains AvgPool1d().
  • My post explains AvgPool2d().
  • My post explains AvgPool3d().

(5) Batch Normalization Layer(2015):

• can normalize input values channel by channel in a batch in CNN to be similar scale to accelerate(speed up) training.
• is unstable with small batch sizes, then it leads to increased train time.
• is used for CNN.
• is not good with RNN.
• is BatchNorm1d(), BatchNorm2d() or BatchNorm3d() in PyTorch: *Memos:
  • Min pooling doesn't exist in PyTorch.
  • BatchNorm1d() is for 1D data.
  • BatchNorm2d() is for 2D data.
  • BatchNorm3d() is for 3D data.
  • My post explains BatchNorm1d().
  • My post explains BatchNorm2d().
  • My post explains BatchNorm3d().

(6) Layer Normalization(2016):

• can normalize input values layer by layer in many types of NN to be similar scale to accelerate training.
• is the improved version of Batch Normalization layer.
• is stable with small batch sizes, then it doesn't lead to increased train time.
• is good with RNN.
• is LayerNorm() in PyTorch. *My post explains LayerNorm().

(7) Dropout Layer(2012):

• can reduce overfitting by randomly dropping out nodes(randomly zeros the elements of a tensor) during training.
• is Dropout() in PyTorch. *My post explains Dropout().

(8) Embedding Layer:

• can convert categorical data to numerical data.
• is used for NLP.
• is Embedding() in PyTorch. *My post explains Embedding().