note

• prompt tuning可看做是prefix tuning的简化版本，在输入层加入prompt tokens，并不需要加入MLP进行调整来解决难训练的问题，作者实验表明随着预训练模型参数量的增加，prompt tuning效果逼近fine tuning效果

一、Propmt tuning

1. peft库中的tuning

• 之前提到过可以借助peft库（Parameter-Efficient Fine-Tuning）进行微调，支持如下tuning：
  • Adapter Tuning（固定原预训练模型的参数 只对新增的adapter进行微调）
  • Prefix Tuning（在输入token前构造一段任务相关的virtual tokens作为prefix，训练时只更新Prefix部分的参数，而Transformer的其他不分参数固定，和构造prompt类似，只是prompt是人为构造的即无法在模型训练时更新参数，而Prefix可以学习<隐式>的prompt）
  • Prompt Tuning（Prefix Tuning的简化版，只在输入层加入prompt tokens，并不需要加入MLP）
  • P-tuning（将prompt转为可学习的embedding层，v2则加入了prompts tokens作为输入）
  • LoRA（Low-Rank Adaption，为了解决adapter增加模型深度而增加模型推理时间、上面几种tuning中prompt较难训练，减少模型的可用序列长度）
    • 该方法可以在推理时直接用训练好的AB两个矩阵和原预训练模型的参数相加，相加结果替换原预训练模型参数。
    • 相当于用LoRA模拟full-tunetune过程

2. prompt tuning怎么搞

• 给出好的prompt可以让LLM生成更好的答案，反过来想通过LLM帮我们找到好的prompt就是prompt tuning的思路，训练让模型看到新的例子生成prompt，并把该段prompt作为前缀拼接到我们自己的prompt上，送入LLM得到结果
  • prompt tuning训练的前缀是向量，所以解释性略差
• 和few show比较：LLM的上下文context长度是有限的（prompt中给出有限的例子，业务复杂时难让模型学习足够多知识），prompt tuning就没有这个限制，只需在训练LLM时给他看足够多例子，之后提问带上一个短的prompt前缀（一般8~20个token）即可
• 和fine tuning比较：prompt tuning是完全冻结LLM模型参数，只需训练一个几个token的prompt前缀；但是fine tuning精调一个模型很耗资源
• 为每一个任务额外添加一个或多个embedding，之后拼接query正常输入LLM，并只训练这些embedding。如下图，左图为单任务全参数微调，右图为prompt tuning。
  • prompt tuning将fine tune任务转为mlm任务。自动学习模板：离散的主要包括 Prompt Mining, Prompt Paraphrasing, Gradient-based Search, Prompt Generation 和 Prompt Scoring；连续的则主要包括Prefix Tuning, Tuning Initialized with Discrete Prompts 和 Hard-Soft Prompt Hybrid Tuning。
  • 正常微调举例：[cls]今天天上都出太阳了，阳光明媚。[SEP]
    prompt输入举例：[cls]今天天气是[MASK]。[SEP] 今天天上都出太阳了，阳光明媚。[SEP]

3. 参数如何选择

prompt tuning论文：The Power of Scale for Parameter-Efficient Prompt Tuning

• 作者的对比实验如下，随着预训练模型参数的增加，很简单的参数设置也能达到不错效果：
  • prompt长度，即下面代码中的num_virtual_tokens参数：模型参数达到一定量级时，Prompt 长度为 1 也能达到不错的效果，Prompt 长度为 20 就能达到极好效果。
  • prompt初始化方式，即下面代码中的prompt_tuning_init：初始化方式中random方式稍差于另外的
  • TaskType任务类型：和peft的其他tuning类似，也有这个参数

class TaskType(str, enum.Enum):
    SEQ_CLS = "SEQ_CLS"   常规分类任务
    SEQ_2_SEQ_LM = "SEQ_2_SEQ_LM" seq2seq任务
    CAUSAL_LM = "CAUSAL_LM"  LM任务
    TOKEN_CLS = "TOKEN_CLS"  token的分类任务：序列标注之类的

二、Prompt tuning代码实战

1. tuning训练

• 数据：twitter_complaints
• 模型：bigscience/bloomz-560m模型
• PromptTuningConfig设置Prompt tuning配置，下面num_virtual_tokens设置prompt前缀的token数，因为token初始化用任务相关文字效果更好，所以下面用Classify if the tweet is a complaint or not:初始化，

#!/usr/bin/env python
# -*- coding: utf-8 -*-
"""
@Author : andy
@Date   : 2023/7/10 20:37
@Contact: 864934027@qq.com 
@File   : prompt_tuning.py 
"""
from transformers import AutoModelForCausalLM, AutoTokenizer, default_data_collator, get_linear_schedule_with_warmup
from peft import get_peft_config, get_peft_model, PromptTuningInit, PromptTuningConfig, TaskType, PeftType
import torch
from datasets import load_dataset
import os
from torch.utils.data import DataLoader
from tqdm import tqdm

device = "mps"
# device = "cuda"
model_name_or_path = "bigscience/bloomz-560m"
tokenizer_name_or_path = "bigscience/bloomz-560m"
peft_config = PromptTuningConfig(
    task_type=TaskType.CAUSAL_LM,
    prompt_tuning_init=PromptTuningInit.TEXT,
    num_virtual_tokens=8,
    prompt_tuning_init_text="Classify if the tweet is a complaint or not:",
    tokenizer_name_or_path=tokenizer_name_or_path,
)

dataset_name = "twitter_complaints"
text_column = "Tweet text"
label_column = "text_label"
max_length = 64
learning_rate = 3e-2
num_epochs = 20
batch_size = 8
output_dir = './output'

# 1. load a subset of the RAFT dataset at https://huggingface.co/datasets/ought/raft
dataset = load_dataset("ought/raft", dataset_name)

# get lable's possible values
label_values = [name.replace("_", "") for name in dataset["train"].features["Label"].names]
# append label value to the dataset to make it more readable
dataset = dataset.map(
    lambda x: {label_column: [label_values[label] for label in x["Label"]]},
    batched=True,
    num_proc=1
)
# have a look at the data structure
dataset["train"][0]

# 2. dataset
tokenizer = AutoTokenizer.from_pretrained(model_name_or_path)
if tokenizer.pad_token_id is None:
    tokenizer.pad_token_id = tokenizer.eos_token_id

def preprocess_fn(examples):
    tweets = examples[text_column]
    # pad labels with a pad token at the end
    labels = [str(x) + tokenizer.pad_token for x in examples[label_column]]
    # concatenate the tweet with it label
    inputs = [f"{text_column} : {tweet}\nLabel :{label}"
              for tweet, label in zip(tweets, labels)]
    # tokenize input
    model_inputs = tokenizer(inputs,
                           padding='max_length',
                           max_length=max_length,
                           truncation=True,)
    # tokenize label, as -100 not a valid token id, do the padding manually here
    labels_input_ids = []
    for i in range(len(labels)):
        ids = tokenizer(labels[i])["input_ids"]
        padding = [-100] * (max_length - len(ids))
        labels_input_ids.append(padding + ids)
        model_inputs["labels"] = labels_input_ids
        # make model inputs tensor
        model_inputs["input_ids"] = [torch.tensor(ids) for ids in model_inputs["input_ids"]]
        model_inputs["attention_mask"] = [torch.tensor(ids) for ids in model_inputs["attention_mask"]]
        model_inputs["labels"] = [torch.tensor(ids) for ids in model_inputs["labels"]]

    return model_inputs

# have a look at the preprocessing result
# print(preprocess_fn(dataset["train"][:2]))

processed_datasets = dataset.map(
    preprocess_fn,
    batched=True,
    num_proc=1,
    remove_columns=dataset["train"].column_names, #remove unprocessed column for training
    load_from_cache_file=False,
    desc="Running tokenizer on datasset"
)

test_size = round(len(processed_datasets["train"]) * 0.2)
train_val = processed_datasets["train"].train_test_split(
    test_size=test_size, shuffle=True, seed=42)
train_data = train_val["train"]
val_data = train_val["test"]


# 3. model
model = AutoModelForCausalLM.from_pretrained(model_name_or_path)
model = get_peft_model(model, peft_config)
print(model.print_trainable_parameters())
trainable params: 8192 || all params: 559222784 || trainable%: 0.0014648902430985358

从上面打印结果看出，模型的参数有5.6亿左右，但是需要训练的参数只占0.001%，只有8192个。

# 4. trainer
from transformers import Trainer, TrainingArguments
trainer = Trainer(
    model=model,
    train_dataset=train_data,
    eval_dataset=val_data,
    data_collator=default_data_collator,
    args=TrainingArguments(
      output_dir='./output',
      per_device_train_batch_size=batch_size,
      num_train_epochs=num_epochs,
      learning_rate=learning_rate,
      load_best_model_at_end=True,
      logging_strategy='steps',
      logging_steps=10,
      evaluation_strategy='steps',
      eval_steps=10,
      save_strategy='steps',
      save_steps=10,
    )
  )
trainer.train()

2. 模型推理比较

# 5. inference
def  inference():
    def generate(inputs, infer_model):
        with torch.no_grad():
            inputs = {k: v.to(device) for k, v in inputs.items()}
            outputs = infer_model.generate(
                input_ids=inputs["input_ids"],
                attention_mask=inputs["attention_mask"],
                max_new_tokens=20,
                eos_token_id=3
            )
            print(tokenizer.batch_decode(outputs.detach().cpu().numpy(), skip_special_tokens=True)[0])

    # (1) base model_inference
    base_model = AutoModelForCausalLM.from_pretrained(model_name_or_path)
    base_model.to(device)
    inputs = tokenizer(
        f'{text_column} : {"@denny the grocery price is soaring, even milk is becoming unaffordable, could you do something?"}\nLabel :',
        return_tensors="pt",  # Return PyTorch torch.Tensor objects.
    )
    generate(inputs, base_model)
    print("----------------------------------------")
    shot1 = f'{text_column} : {"@nationalgridus I have no water and the bill is current and paid. Can you do something about this?"}\nLabel :complaint\n'
    shot2 = f'{text_column} : {"@HMRCcustomers No this is my first job"}\nLabel :no complaint\n'
    input = f'{text_column} : {"@denny the grocery price is soaring, even milk is becoming unaffordable, could you do something?"}\nLabel :'
    inputs_few_shot = tokenizer(
        shot1 + shot2 + input,
        return_tensors="pt",
    )
    generate(inputs_few_shot, base_model)

    # (2) prompt-tuned model_inference
    from peft import PeftModel, PeftConfig
    path = "/content/drive/MyDrive/prompt_tuning"
    config = PeftConfig.from_pretrained(path)
    pretrained_model = AutoModelForCausalLM.from_pretrained(config.base_model_name_or_path)
    prompt_tuned_model = PeftModel.from_pretrained(pretrained_model, path)
    prompt_tuned_model.to(device)
    inputs = tokenizer(
        f'{text_column} : {"@denny the grocery price is soaring, even milk is becoming unaffordable, could you do something?"}\nLabel :',
        return_tensors="pt",  # Return PyTorch torch.Tensor objects.
    )
    generate(inputs, prompt_tuned_model)

inference()

• 上面base model推理结果：

Tweet text : @denny the grocery price is soaring, even milk is becoming unaffordable, could you do something?
Label : @denny the grocery<?php
/**
 * Copyright © 2016 Google Inc.

----------------------------------------
Tweet text : @nationalgridus I have no water and the bill is current and paid. Can you do something about this?
Label :complaint
Tweet text : @HMRCcustomers No this is my first job
Label :no complaint
Tweet text : @denny the grocery price is soaring, even milk is becoming unaffordable, could you do something?
Label :complaint<?php
/**
 * Copyright © Magento, Inc. All rights reserved.

• prompt-tuned model推理结果：

Tweet text : @denny the grocery price is soaring, even milk is becoming unaffordable, could you do something?
Label :complaint

3. 其他tuning技术

• prefix tuning和prompt tuning都不需要改LLM模型参数本身，但prefix tuning不进在用户输入该层找到一个前缀，还要在LLM的每层都找到一个前缀并加上，训练成本明显更高
• p-tuning则不仅可在用户输入的开头加附加信息，也可以在中间或结尾附加信息
• lora tuning如下图，上一篇博客也讲过

Reference

[1] https://github.com/jxhe/unify-parameter-efficient-tuning
[2] Continuous Optimization：从Prefix-tuning到更强大的P-Tuning V2
[3] 五万字综述！Prompt-Tuning：深度解读一种新的微调范式
[4] 还在Fine-tune大规模预训练模型？了解下Prompt-tuning
[5] 让天下没有难Tuning的大模型：PEFT技术简介.阿里-风飏
[6] prompt tuning论文：The Power of Scale for Parameter-Efficient Prompt Tuning
[6] 你还弄不清xxxForCausalLM和xxxForConditionalGeneration吗？