从零实现一个大模型（一）

#LLM #Python #BPE #GPT #Transformer

学习

本文从零实现一个大模型，用于帮助理解大模型的基本原理，本文是一个读书笔记，内容来自于Build a Large Language Model (From Scratch)

处理文本

LLM模型首先要处理文本，处理文本的目标是将文本转换成高维度的嵌入向量，具有相关性的文本的距离是相近的。高维度坐标无法掩饰，这里使用二维的坐标做演示，具有相似属性的文本在二维的平面坐标系里考的更近。

但是二维的向量所能包含的信息太少了，LLM通常使用上千维度的向量，这也是LLM消耗硬件资源的原因之一。

分词器 Tokenizer

在处理文本之前需要将，需要使用分词器将文本分割成更小的单元，比如英文句子中的单词和标点符号

以下代码将一篇txt格式的短篇小说划分成单词和标点符号，我们称之为token。

import re

class SimpleTokenizerV1:
    def __init__(self, vocab):
        self.str_to_int = vocab
        self.int_to_str = {i:s for s,i in vocab.items()}
    
    def encode(self, text):
        preprocessed = re.split(r'([,.:;?_!"()\']|--|\s)', text)
                                
        preprocessed = [
            item.strip() for item in preprocessed if item.strip()
        ]
        ids = [self.str_to_int[s] for s in preprocessed]
        return ids
        
    def decode(self, ids):
        text = " ".join([self.int_to_str[i] for i in ids])
        # 使用空格分割
        text = re.sub(r'\s+([,.?!"()\'])', r'\1', text)
        return text

with open("the-verdict.txt", "r", encoding="utf-8") as f:
    raw_text = f.read() 
print("Total number of character:", len(raw_text))
print(raw_text[:99])

Total number of character: 20479
I HAD always thought Jack Gisburn rather a cheap genius--though a good fellow enough--so it was no

将token去重后组成一个词库vocab，词库中的每个元素都有一个唯一个ID，称之为token ID

preprocessed = re.split(r'([,.:;?_!"()\']|--|\s)', raw_text)
preprocessed = [item.strip() for item in preprocessed if item.strip()]
# print(preprocessed[:30])
all_words = sorted(set(preprocessed))
vocab_size = len(all_words)
print(vocab_size)
vocab = {token:integer for integer,token in enumerate(all_words)}

for i, item in enumerate(vocab.items()):
    print(item)
    if i >= 50:
        break

('!', 0)
('"', 1)
("'", 2)
('(', 3)
(')', 4)
(',', 5)
('--', 6)
('.', 7)
(':', 8)
(';', 9)
('?', 10)
('A', 11)
('Ah', 12)
('Among', 13)
('And', 14)
('Are', 15)
('Arrt', 16)
('As', 17)
('At', 18)
('Be', 19)
('Begin', 20)
('Burlington', 21)
('But', 22)
('By', 23)
('Carlo', 24)
('Chicago', 25)
('Claude', 26)
('Come', 27)
('Croft', 28)
('Destroyed', 29)
('Devonshire', 30)
('Don', 31)
('Dubarry', 32)
('Emperors', 33)
('Florence', 34)
('For', 35)
('Gallery', 36)
('Gideon', 37)
('Gisburn', 38)
('Gisburns', 39)
('Grafton', 40)
('Greek', 41)
('Grindle', 42)
('Grindles', 43)
('HAD', 44)
('Had', 45)
('Hang', 46)
('Has', 47)
('He', 48)
('Her', 49)
('Hermia', 50)

使用上面的词库，将下面的文本转换为token ID：

tokenizer = SimpleTokenizerV1(vocab)

text = """"It's the last he painted, you know," 
           Mrs. Gisburn said with pardonable pride."""
ids = tokenizer.encode(text)
print(ids)

[1, 56, 2, 850, 988, 602, 533, 746, 5, 1126, 596, 5, 1, 67, 7, 38, 851, 1108, 754, 793, 7]

1	tokenizer.decode(ids) # 将token ID转化为文本

'" It\' s the last he painted, you know," Mrs. Gisburn said with pardonable pride.'

使用上面的方法无法处理词库里没有的单词，所以我们使用占位符来处理不认识的单词，修改上面的代码如下：

class SimpleTokenizerV2:
    def __init__(self, vocab):
        self.str_to_int = vocab
        self.int_to_str = { i:s for s,i in vocab.items()}

    def encode(self, text):
        preprocessed = re.split(r'([,.?_!"()\']|--|\s)', text)
        preprocessed = [item.strip() for item in preprocessed if item.strip()]
        preprocessed = [item if item in self.str_to_int else "<|unk|>" for item in preprocessed]

        ids = [self.str_to_int[s] for s in preprocessed]
        return ids

    def decode(self, ids):
        text = " ".join([self.int_to_str[i] for i in ids])

        text = re.sub(r'\s+([,.?!"()\'])', r'\1', text)                    #B
        return text

all_tokens = sorted(list(set(preprocessed)))
all_tokens.extend(["<|endoftext|>", "<|unk|>"])
vocab = {token:integer for integer,token in enumerate(all_tokens)}
len(vocab.items())

可以看到词库拓展了两个新的token，分别为<|endoftext|>和<|unk|>，词库中不存在的token会被转化为<|unk|>，token ID是1131

1 2	for i, item in enumerate(list(vocab.items())[-5:]): print(item)

('younger', 1127)
('your', 1128)
('yourself', 1129)
('<|endoftext|>', 1130)
('<|unk|>', 1131)

tokenizer = SimpleTokenizerV2(vocab)

text1 = "Hello, do you like tea?"
text2 = "In the sunlit terraces of the palace."

text = " <|endoftext|> ".join((text1, text2))

print(text)

Hello, do you like tea? <|endoftext|> In the sunlit terraces of the palace.

1	tokenizer.encode(text)

[1131, 5, 355, 1126, 628, 975, 10, 1130, 55, 988, 956, 984, 722, 988, 1131, 7]

1	tokenizer.decode(tokenizer.encode(text))

'<|unk|>, do you like tea? <|endoftext|> In the sunlit terraces of the <|unk|>.'

BPE字节对编码

上面的例子为了演示，直接采用了比较简单的策略进行分词，实际过程中并不会采用这种方式，GPT2使用了字节对编码作为分词器（BytePair encoding，简称BPE），该分词器会将文本分割成更小的单元，如unfamiliarword别分割成[“unfam”, “iliar”, “word”]

分词器的原理可以参考这里，本文使用了OpenAI使用Rust实现的开源库tiktoken，它有好的的性能。

使用tiktoken处理同样的文本的结果如下：

import tiktoken
tokenizer = tiktoken.get_encoding("gpt2")
text = (
    "Hello, do you like tea? <|endoftext|> In the sunlit terraces"
     "of someunknownPlace."
)

integers = tokenizer.encode(text, allowed_special={"<|endoftext|>"})
print(integers)

[15496, 11, 466, 345, 588, 8887, 30, 220, 50256, 554, 262, 4252, 18250, 8812, 2114, 1659, 617, 34680, 27271, 13]

1 2	strings = tokenizer.decode(integers) print(strings)

Hello, do you like tea? <|endoftext|> In the sunlit terracesof someunknownPlace.

使用滑动窗口处理数据样本

LLM每次生成一个词，所以在训练时我们需要预处理文本，处理方式如下图，红色的是目标值，蓝色的试输入值。这种方法称为滑动窗口。

with open("the-verdict.txt", "r", encoding="utf-8") as f:
    raw_text = f.read()

enc_text = tokenizer.encode(raw_text)
print(len(enc_text))

enc_sample = enc_text[50:]
context_size = 4

x = enc_sample[:context_size] # 输入值
y = enc_sample[1:context_size+1] # 使用滑动窗口的策略

print(f"x: {x}")
print(f"y:      {y}")

x: [290, 4920, 2241, 287]
y:      [4920, 2241, 287, 257]

上面的代码如下图所示，蓝色的是输入，红色的目标

for i in range(1, context_size+1):
    context = enc_sample[:i]
    desired = enc_sample[i]

    print(context, "---->", desired)

[290] ----> 4920
[290, 4920] ----> 2241
[290, 4920, 2241] ----> 287
[290, 4920, 2241, 287] ----> 257

for i in range(1, context_size+1):
    context = enc_sample[:i]
    desired = enc_sample[i]

    print(tokenizer.decode(context), "---->", tokenizer.decode([desired]))

 and ---->  established
 and established ---->  himself
 and established himself ---->  in
 and established himself in ---->  a

接下来构建一个类来处理训练数据集，先使用分词器处理输入的文本数据，将他们转换为token id，然后使用滑动窗口的策略将数据输入数据input_ids和目标数据target_ids。

import torch
from torch.utils.data import Dataset, DataLoader

class GPTDatasetV1(Dataset):
    def __init__(self, txt, tokenizer, max_length, stride):
        self.input_ids = []
        self.target_ids = []

        # 分词器处理文本
        token_ids = tokenizer.encode(txt, allowed_special={"<|endoftext|>"})
        assert len(token_ids) > max_length, "Number of tokenized inputs must at least be equal to max_length+1"

        # 使用滑动窗口将数据样本分成两部分：输入值和目标值
        for i in range(0, len(token_ids) - max_length, stride):
            input_chunk = token_ids[i:i + max_length]
            target_chunk = token_ids[i + 1: i + max_length + 1]
            self.input_ids.append(torch.tensor(input_chunk))
            self.target_ids.append(torch.tensor(target_chunk))

    def __len__(self):
        return len(self.input_ids)

    def __getitem__(self, idx):
        return self.input_ids[idx], self.target_ids[idx]

def create_dataloader_v1(txt, batch_size=4, max_length=256, 
                         stride=128, shuffle=True, drop_last=True,
                         num_workers=0):

    # 初始化分词器
    tokenizer = tiktoken.get_encoding("gpt2")

    # Create dataset
    dataset = GPTDatasetV1(txt, tokenizer, max_length, stride)

    # Create dataloader
    dataloader = DataLoader(
        dataset,
        batch_size=batch_size,
        shuffle=shuffle,
        drop_last=drop_last,
        num_workers=num_workers
    )

    return dataloader

with open("the-verdict.txt", "r", encoding="utf-8") as f:
    raw_text = f.read()

# 滑动窗口的步长为1时
dataloader = create_dataloader_v1(raw_text, batch_size=8, max_length=4, stride=1, shuffle=False)

data_iter = iter(dataloader)
inputs, targets = next(data_iter)
print("Inputs:\n", inputs)
print("\nTargets:\n", targets)

Inputs:
 tensor([[   40,   367,  2885,  1464],
        [  367,  2885,  1464,  1807],
        [ 2885,  1464,  1807,  3619],
        [ 1464,  1807,  3619,   402],
        [ 1807,  3619,   402,   271],
        [ 3619,   402,   271, 10899],
        [  402,   271, 10899,  2138],
        [  271, 10899,  2138,   257]])

Targets:
 tensor([[  367,  2885,  1464,  1807],
        [ 2885,  1464,  1807,  3619],
        [ 1464,  1807,  3619,   402],
        [ 1807,  3619,   402,   271],
        [ 3619,   402,   271, 10899],
        [  402,   271, 10899,  2138],
        [  271, 10899,  2138,   257],
        [10899,  2138,   257,  7026]])

# 滑动窗口的步长为4时
dataloader = create_dataloader_v1(raw_text, batch_size=8, max_length=4, stride=4, shuffle=False)

data_iter = iter(dataloader)
inputs, targets = next(data_iter)
print("Inputs:\n", inputs)
print("\nTargets:\n", targets)

Inputs:
 tensor([[   40,   367,  2885,  1464],
        [ 1807,  3619,   402,   271],
        [10899,  2138,   257,  7026],
        [15632,   438,  2016,   257],
        [  922,  5891,  1576,   438],
        [  568,   340,   373,   645],
        [ 1049,  5975,   284,   502],
        [  284,  3285,   326,    11]])

Targets:
 tensor([[  367,  2885,  1464,  1807],
        [ 3619,   402,   271, 10899],
        [ 2138,   257,  7026, 15632],
        [  438,  2016,   257,   922],
        [ 5891,  1576,   438,   568],
        [  340,   373,   645,  1049],
        [ 5975,   284,   502,   284],
        [ 3285,   326,    11,   287]])

上面步长stride为1和4的示意图如下：

创建嵌入向量

将数据分成输入值和目标值之后，接下来的步骤就是要token ID转化为向量以便LLM做后续处理，这个步骤称之为嵌入层

将token ID转化为向量可以使用torch.tensor方法，下面是一个示例：

import torch
input_ids = torch.tensor([2, 3, 5, 1])
vocab_size = 6 # 定义词库的大小
output_dim = 3 # 定义每个token ID输出的维度

torch.manual_seed(123)
embedding_layer = torch.nn.Embedding(vocab_size, output_dim) #嵌入层
print(embedding_layer.weight) # 打印嵌入层
print(embedding_layer(input_ids)) # 使用嵌入层处理输入数据

Parameter containing:
tensor([[ 0.3374, -0.1778, -0.1690],
        [ 0.9178,  1.5810,  1.3010],
        [ 1.2753, -0.2010, -0.1606],
        [-0.4015,  0.9666, -1.1481],
        [-1.1589,  0.3255, -0.6315],
        [-2.8400, -0.7849, -1.4096]], requires_grad=True)
tensor([[ 1.2753, -0.2010, -0.1606],
        [-0.4015,  0.9666, -1.1481],
        [-2.8400, -0.7849, -1.4096],
        [ 0.9178,  1.5810,  1.3010]], grad_fn=<EmbeddingBackward0>)

可以看到嵌入层是二维向量，长度为6，每个元素都是一个三维的向量，使用嵌入层处理输入值后，是一个长度为4的二维向量，每个token ID都被转换为一个三维向量。

位置编码

上面将token ID转化成嵌入向量时，没有包含子词的位置信息，同一个单词在句子的不同位置所代表的含义可能是不同的，所以我们还需要把token ID的位置信息也加入到向量中。

# 创建嵌入层
vocab_size = 50257 # BPE编码的词库长度微50257
output_dim = 256 # 将向量转化为256维
token_embedding_layer = torch.nn.Embedding(vocab_size, output_dim) # 创建token嵌入层

max_length = 4
dataloader = create_dataloader_v1(
    raw_text, batch_size=8, max_length=max_length,
    stride=max_length, shuffle=False
)
data_iter = iter(dataloader)
inputs, targets = next(data_iter)
token_embeddings = token_embedding_layer(inputs)
print(token_embeddings.shape)

torch.Size([8, 4, 256])

以上结果表明inputs有8个批次的数据，每个批次有4个token，每个token被转换成一个256纬度的向量

接下来获取位置向量

# 处理位置向量
context_length = max_length
pos_embedding_layer = torch.nn.Embedding(context_length, output_dim) # 创建位置嵌入层
pos_embeddings = pos_embedding_layer(torch.arange(max_length)) # 生成和位置相关的嵌入向量

print(pos_embeddings.shape)

torch.Size([4, 256])

为了简单起见，这里简单的将toke ID的向量和位置向量相加得到最终的输入向量

1 2	input_embeddings = token_embeddings + pos_embeddings print(input_embeddings.shape)

torch.Size([8, 4, 256])

这样我们就得到了嵌入向量，完整的代码如下，后面我们要使用这个类处理所有的输入数据。

import tiktoken
import torch
from torch.utils.data import Dataset, DataLoader

class GPTDatasetV1(Dataset):
    def __init__(self, txt, tokenizer, max_length, stride):
        self.input_ids = []
        self.target_ids = []

        # Tokenize the entire text
        token_ids = tokenizer.encode(txt, allowed_special={"<|endoftext|>"})

        # Use a sliding window to chunk the book into overlapping sequences of max_length
        for i in range(0, len(token_ids) - max_length, stride):
            input_chunk = token_ids[i:i + max_length]
            target_chunk = token_ids[i + 1: i + max_length + 1]
            self.input_ids.append(torch.tensor(input_chunk))
            self.target_ids.append(torch.tensor(target_chunk))

    def __len__(self):
        return len(self.input_ids)

    def __getitem__(self, idx):
        return self.input_ids[idx], self.target_ids[idx]


def create_dataloader_v1(txt, batch_size, max_length, stride,
                         shuffle=True, drop_last=True, num_workers=0):
    # Initialize the tokenizer
    tokenizer = tiktoken.get_encoding("gpt2")

    # Create dataset
    dataset = GPTDatasetV1(txt, tokenizer, max_length, stride)

    # Create dataloader
    dataloader = DataLoader(
        dataset, batch_size=batch_size, shuffle=shuffle, drop_last=drop_last, num_workers=num_workers)

    return dataloader

with open("the-verdict.txt", "r", encoding="utf-8") as f:
    raw_text = f.read()

vocab_size = 50257
output_dim = 256
context_length = 1024


token_embedding_layer = torch.nn.Embedding(vocab_size, output_dim)
pos_embedding_layer = torch.nn.Embedding(context_length, output_dim)

batch_size = 8
max_length = 4
dataloader = create_dataloader_v1(
    raw_text,
    batch_size=batch_size,
    max_length=max_length,
    stride=max_length
)

for batch in dataloader:
    x, y = batch

    token_embeddings = token_embedding_layer(x)
    pos_embeddings = pos_embedding_layer(torch.arange(max_length))

    input_embeddings = token_embeddings + pos_embeddings

    break

1	print(input_embeddings.shape)

torch.Size([8, 4, 256])

参考资料

Build a Large Language Model (From Scratch)

从零实现一个大模型（一）

目录

1. 文本处理

2. 注意力机制

3. 开发一个Transform架构的大模型

4. 使用无标记数据预训练模型

5. 分类微调

6. 指令微调