This article is for those who have deeply understood the mathematical principles and operation process of RNN, LSTM and GRU. If they do not understand its basic idea and process, it may not be very simple to understand.

## 1, Let's start with an example

This is an example on the official website. Taking LSTM as an example this time, in fact, the operation process of GRU, LSTM and RNN is very similar.

import torch import torch.nn as nn lstm = nn.LSTM( 10, 20, 2) # Sequence length seq_len=5, batch_size=3, data vector dimension = 10 input = torch.randn( 5, 3, 10) # The dimensions of the initialized hidden element and memory element are usually the same # 2 LSTM layers, batch_size=3, hide meta dimension 20 h0 = torch.randn( 2, 3, 20) c0 = torch.randn( 2, 3, 20) # There are two layers of lstm, and output is the output of each word vector of the last layer of lstm corresponding to the hidden layer, which is independent of the number of layers and only related to the sequence length # hn,cn are the output of the last hidden element and memory element of all layers output, (hn, cn) = lstm( input, (h0, c0)) print(output.size(),hn.size(),cn.size()) # namely: # torch.Size([5, 3, 20]) # torch.Size([2, 3, 20]) # torch.Size([2, 3, 20]))

Later, I will explain the above operation process in detail. Let's take a look at the definition of LSTM, which is a class

## 2, Definition of LSTM class

class LSTM( RNNBase): '''parameter Args: input_size: The feature dimension of the input data. For example, if I model a time series, the feature is 1. If I model a sentence, the embedded vector of each word is 10, it is 10 hidden_size: That is, the number of hidden nodes in the recurrent neural network. This is self-defined. It can be as many as you want, as will be described in detail later num_layers: Stacked LSTM The number of layers is one by default, or you can define it yourself Default: 1 bias: LSTM Whether the layer uses the offset matrix, and the offset weight is `b_ih` and `b_hh`. Default: ``True``((used by default) batch_first: If set ``True``, then the input and output tensors are provided as (batch, seq, feature). Default: ``False``，(seq,batch,features) dropout: Whether to use dropout Mechanism. The default is 0, which means it is not used dropout，If a non-zero number is provided, it means that in each LSTM Default after layer dropout，But the last layer LSTM Layer not used dropout. bidirectional: Is it bidirectional RNN，The default is yes or no, If ``True``, becomes a bidirectional LSTM. Default: ``False`` #--------------------------------------------------------------------------------------- The input to the constructor of class is Inputs: input, (h_0, c_0) - **input** of shape `(seq_len, batch, input_size)`: tensor containing the features of the input sequence. - **h_0** of shape `(num_layers * num_directions, batch, hidden_size)`: tensor containing the initial hidden state for each element in the batch. If the LSTM is bidirectional, num_directions should be 2, else it should be 1. - **c_0** of shape `(num_layers * num_directions, batch, hidden_size)`: tensor containing the initial cell state for each element in the batch. If `(h_0, c_0)` is not provided, both **h_0** and **c_0** default to zero. #---------------------------------------------------------------------------------- What is the output: Outputs: output, (h_n, c_n) - **output** of shape `(seq_len, batch, num_directions * hidden_size)`: tensor containing the output features `(h_t)` from the last layer of the LSTM, for each `t`. If a :class:`torch.nn.utils.rnn.PackedSequence` has been given as the input, the output will also be a packed sequence. For the unpacked case, the directions can be separated using ``output.view(seq_len, batch, num_directions, hidden_size)``, with forward and backward being direction `0` and `1` respectively. Similarly, the directions can be separated in the packed case. - **h_n** of shape `(num_layers * num_directions, batch, hidden_size)`: tensor containing the hidden state for `t = seq_len`. Like *output*, the layers can be separated using ``h_n.view(num_layers, num_directions, batch, hidden_size)`` and similarly for *c_n*. - **c_n** of shape `(num_layers * num_directions, batch, hidden_size)`: tensor containing the cell state for `t = seq_len`. #------------------------------------------------------------------------------------------ The properties of the class are Attributes: weight_ih_l[k] : the learnable input-hidden weights of the :math:`\text{k}^{th}` layer `(W_ii|W_if|W_ig|W_io)`, of shape `(4*hidden_size, input_size)` for `k = 0`. Otherwise, the shape is `(4*hidden_size, num_directions * hidden_size)` weight_hh_l[k] : the learnable hidden-hidden weights of the :math:`\text{k}^{th}` layer `(W_hi|W_hf|W_hg|W_ho)`, of shape `(4*hidden_size, hidden_size)` bias_ih_l[k] : the learnable input-hidden bias of the :math:`\text{k}^{th}` layer `(b_ii|b_if|b_ig|b_io)`, of shape `(4*hidden_size)` bias_hh_l[k] : the learnable hidden-hidden bias of the :math:`\text{k}^{th}` layer `(b_hi|b_hf|b_hg|b_ho)`, of shape `(4*hidden_size)` '''

The above parameters are a little too many. I won't translate them one by one. In fact, they are very easy to understand. Each of them is relatively clear.

## 3, In depth understanding of required parameters

1. The three parameters of the constructors of RNN, GRU and LSTM must be understood -- step 1: construct the loop layer object

When creating a circular layer, the first step is to construct a circular layer, as follows:

lstm = nn.LSTM(10, 20, 2)

The parameter list of the constructor is as follows:

class LSTM( RNNBase): '''parameter Args: input_size: hidden_size: num_layers: bias: batch_first: dropout: bidirectional: '''

(1)input_size: refers to the characteristic dimension of each word. For example, if I have a sentence in which each word is represented by a 10-dimensional vector, then input_size is 10;

(2)hidden_size: refers to the number of hidden nodes of each LSTM internal unit in the loop layer. This is self-defined and can be set arbitrarily;

(3)num_layers: the number of layers of the circular layer. The default is one layer. This depends on your own situation.

For example:

There is only one circular layer on the left and two circular layers on the right.

2. Construct the forward propagation process through the object constructed in the first step - step 2: call the loop layer object, pass in parameters, and get the return value

The general operations are as follows:

output, (hn, cn) = lstm(input, (h0, c0))

Take LSTM as an example,

(1) Input parameters

Input: must be in this format (seq,batch,feature). The first seq refers to the length of the sequence, which is determined according to my own data. For example, the maximum length of my sentence is 20 words, so here is 20. The above example assumes that the sentence length is 5; The second is batch, which is easy to understand. It is to use several samples at a time, such as three groups of samples; The third feature refers to the vector dimension of each word. It should be noted that this must be consistent with the first parameter input of the constructor_ The size remains the same. In the above example, it is 10

(h0,c0): refers to the initial state of each cycle layer. It can be initialized to 0 without specifying. Here, because LSTM has two states to be transferred, there are two. For example, if ordinary RNN and GRU have only one state to be transferred, only one h state needs to be transferred, as shown below:

output, hn = rnn( input, h0) # Ordinary rnn output, hn = gru( input, h0) # gru

It should be noted here that the dimension of the passed in status parameter is still in terms of LSTM:

The data dimensions of h0 and c0 are (num_layers * num_directions, batch, hidden_size). What does this mean?

First num_layer refers to the base circulating layer. It is easy to understand that several layers should have several initial states;

Second num_directions refers to whether the loop layer is bidirectional (specified by the bidirectional parameter in the constructor). If it is not bidirectional, the value is 1; if it is bidirectional, the value is 2;

The third batch refers to the batch of each data, which can be consistent with the previous batch;

Last hidden_size refers to the number of hidden nodes in each node of the loop layer. This requires a good understanding of the whole operation process of the loop neural network!

(2) Output results

In fact, the output results match the input results, as follows:

output, hn = rnn( input, h0) # Ordinary rnn output, hn = gru( input, h0) # gru output, (hn, cn) = lstm( input, (h0, c0)) # lstm

In terms of lstm:

The output dimension of output: (seq_len, batch, num_directions * hidden_size). In the above example, it should be (5,3,20). We verified that it is true. It should be noted that the first dimension is SEQ_ Len, that is, the output at each time point is the output result, which is different from the hidden layer;

The output dimensions of hn and cn are (num_layers * num_directions, batch, hidden_size) and (2,3,20) in the above example, which have also been verified. We find that this is consistent with the sequence length seq_len doesn't matter. Why? The output state only refers to the output state of the last loop layer node.

As shown in the figure below:

The following example is drawn with an ordinary RNN, so there is only one state h and no state c.

3. Several important attributes are understood

Whether it is RNN, GRU or lstm, the internal learnable parameters are actually several weight matrices, including offset matrices. How do you view these learned parameters? It is realized through these matrices

(1)weight_ih_l[k]: This represents the weight matrix input between hidden layers, where K represents the cycle layer,

If K=0, it represents the matrix from the lowest input layer to the first cycle layer, and the dimension is (hidden_size, input_size). If k > 0, it represents the weight matrix from the first cycle layer to the second cycle layer, the second cycle layer to the third cycle layer, and so on. The shape is (hidden_size, num_directions * hidden_size).

(2)weight_hh_l[k]: refers to the weight matrix between the circulating layers. Here, K refers to the circulating layer, and the values are 0,1,2,3,4. Shape is (hidden_size, hidden_size)

Note: the number of layers of the circulating layer starts from 0, 0 represents the first circulating layer, 1 represents the second circulating layer, and so on.

(3)bias_ih_l[k]: the offset item of the kth cycle layer, which represents the offset input to the cycle layer, and the dimension is (hidden_size)

(4)bias_hh_l[k]: the offset item of the kth cycle layer represents the offset from the cycle layer to the interior of the cycle layer, and the dimension is (hidden_size).

# First, import the relevant modules required by RNN import torch import torch.nn as nn # Data vector dimension 10, hidden meta dimension 20, two RNN layers in series (if it is 1, it can be omitted, and the default is 1) rnn = nn.RNN( 10, 20, 2) # Sequence length seq_len=5, batch_size=3, data vector dimension = 10 input = torch.randn( 5, 3, 10) # The dimensions of the initialized hidden element and memory element are usually the same # 2 RNN layers, batch_size=3, hide meta dimension 20 h0 = torch.randn( 2, 3, 20) # There are two layers of RNN. Output is the output of each word vector of the last layer RNN corresponding to the hidden layer, which is independent of the number of layers and only related to the sequence length # hn,cn are the output of the last hidden element and memory element of all layers output, hn = rnn( input, h0) print(output.size(),hn.size()) # They are: torch. Size ([5, 3, 20]) torch. Size ([2, 3, 20]) # Take a look at some important attributes: print( "------------input-->hide------------------------------") print(rnn.weight_ih_l0.size()) print(rnn.weight_ih_l1.size()) print(rnn.bias_ih_l0.size()) print(rnn.bias_ih_l1.size()) print( "------------hide-->hide------------------------------") print(rnn.weight_hh_l0.size()) print(rnn.weight_hh_l1.size()) print(rnn.bias_hh_l0.size()) print(rnn.bias_hh_l1.size()) '''The output result is: ------------input-->hide------------------------------ torch.Size([20, 10]) torch.Size([20, 20]) torch.Size([20]) torch.Size([20]) ------------hide-->hide------------------------------ torch.Size([20, 20]) torch.Size([20, 20]) torch.Size([20]) torch.Size([20]) '''

As like as two peas, we found the result is exactly the same as the one described.

Reference article:

https://blog.csdn.net/lwgkzl/article/details/88717678

https://blog.csdn.net/rogerfang/article/details/84500754

https://blog.csdn.net/qq_27825451/article/details/99691258