因为之前在项目中一直使用Tensorflow，最近需要处理NLP问题，对Pytorch框架还比较陌生，所以特地再学习一下pytorch在自然语言处理问题中的简单使用，这里做一个记录。

一、Pytorch基础

首先，第一步是导入pytorch的一系列包

import torch
import torch.autograd as autograd #Autograd为Tensor所有操作提供自动求导方法
import torch.nn as nn
import torch.nn.functional as F
import torch.optim as optim

1）Tensor张量

a) 创建Tensors

#tensor
x = torch.Tensor([[1,2,3],[4,5,6]])
#size为2x3x4的随机数随机数
x = torch.randn((2,3,4))

b) Tensors计算

x = torch.Tensor([[1,2],[3,4]])
y = torch.Tensor([[5,6],[7,8]])
z = x+y

c) Reshape Tensors

x = torch.randn(2,3,4)
#拉直
x = x.view(-1)
#4*6维度
x = x.view(4,6)

2）计算图和自动微分

a) Variable变量

#将Tensor变为Variable
x = autograd.Variable(torch.Tensor([1,2,3]),requires_grad = True)
#将Variable变为Tensor
y = x.data

b) 反向梯度算法

x = autograd.Variable(torch.Tensor([1,2]),requires_grad=True)
y = autograd.Variable(torch.Tensor([3,4]),requires_grad=True)
z = x+y
#求和
s = z.sum()
#反向梯度传播
s.backward()
print(x.grad)

c) 线性映射

linear = nn.Linear(3,5) #三维线性映射到五维
x = autograd.Variable(torch.randn(4,3))
#输出为（4,5）维
y = linear(x)

d) 非线性映射（激活函数的使用）

x = autograd.Variable(torch.randn(5))
#relu激活函数
x_relu = F.relu(x)
print(x_relu)
x_soft = F.softmax(x)
#softmax激活函数
print(x_soft)
print(x_soft.sum())

output:

Variable containing:
-0.9347
-0.9882
 1.3801
-0.1173
 0.9317
[torch.FloatTensor of size 5]
 
Variable containing:
 0.0481
 0.0456
 0.4867
 0.1089
 0.3108
[torch.FloatTensor of size 5]
 
Variable containing:
 1
[torch.FloatTensor of size 1]
 
Variable containing:
-3.0350
-3.0885
-0.7201
-2.2176
-1.1686
[torch.FloatTensor of size 5]

二、Pytorch创建网络

1) word embedding词嵌入

通过nn.Embedding(m,n)实现，m表示所有的单词数目，n表示词嵌入的维度。

word_to_idx = {'hello':0,'world':1}
embeds = nn.Embedding(2,5) #即两个单词，单词的词嵌入维度为5
hello_idx = torch.LongTensor([word_to_idx['hello']])
hello_idx = autograd.Variable(hello_idx)
hello_embed = embeds(hello_idx)
print(hello_embed)

output:

Variable containing:
-0.6982 0.3909 -1.0760 -1.6215 0.4429
[torch.FloatTensor of size 1x5]

2) N-Gram 语言模型

先介绍一下N-Gram语言模型，给定一个单词序列 ，计算 ，其中 是序列的第 个单词。

import torch
import torch.nn as nn
import torch.nn.functional as F
import torch.autograd as autograd
import torch.optim as optim
 
from six.moves import xrange

对句子进行分词：

context_size = 2
embed_dim = 10
text_sequence = """When forty winters shall besiege thy brow,
And dig deep trenches in thy beauty's field,
Thy youth's proud livery so gazed on now,
Will be a totter'd weed of small worth held:
Then being asked, where all thy beauty lies,
Where all the treasure of thy lusty days;
To say, within thine own deep sunken eyes,
Were an all-eating shame, and thriftless praise.
How much more praise deserv'd thy beauty's use,
If thou couldst answer 'This fair child of mine
Shall sum my count, and make my old excuse,'
Proving his beauty by succession thine!
This were to be new made when thou art old,
And see thy blood warm when thou feel'st it cold.""".split()
#分词
trigrams = [ ([text_sequence[i], text_sequence[i+1]], text_sequence[i+2]) for i in xrange(len(text_sequence) - 2) ]
trigrams[:10]