使用 TRL 训练Reward Model奖励模型

训练过程

在此示例中，将微调“distilroberta-base”模型。
该formatting_func函数将指令与所选和拒绝的响应相结合，创建两个新字符串。这些字符串被标记化，成为奖励模型的输入，该模型根据这些示例学习区分好响应和坏响应。损失函数的设计方式是最大化所选和拒绝响应的分数之间的差异。使用 trl 的 RewardTrainer 来微调基础模型。它是该类的子类，transformers.Trainer并继承了其所有属性和方法。

#Select a base model whch we need to train for reward modeling.
model_name = "distilroberta-base"
model = AutoModelForSequenceClassification.from_pretrained(model_name, num_labels=1)
tokenizer = AutoTokenizer.from_pretrained(model_name)
if tokenizer.pad_token is None:
    tokenizer.pad_token = tokenizer.eos_token
    model.config.pad_token_id = model.config.eos_token_id


def formatting_func(examples):
    kwargs = {"padding": "max_length", "truncation": True, "max_length": 512, "return_tensors": "pt"}
    prompt_plus_chosen_response = examples["instruction"] + "\n" + examples["chosen_response"]
    prompt_plus_rejected_response = examples["instruction"] + "\n" + examples["rejected_response"]
    tokens_chosen = tokenizer.encode_plus(prompt_plus_chosen_response, **kwargs)
    tokens_rejected = tokenizer.encode_plus(prompt_plus_rejected_response, **kwargs)
    return {
        "input_ids_chosen": tokens_chosen["input_ids"][0], "attention_mask_chosen": tokens_chosen["attention_mask"][0],
        "input_ids_rejected": tokens_rejected["input_ids"][0], "attention_mask_rejected": tokens_rejected["attention_mask"][0]
    }
formatted_dataset = prepared_dataset.map(formatting_func)
formatted_dataset = formatted_dataset.train_test_split()
# Configuring the training arguments
training_args = TrainingArguments(
    output_dir="./reward_model",
    per_device_train_batch_size=16,
    evaluation_strategy="steps",
    logging_steps=1,
    num_train_epochs = 10,
    report_to=None,
)
# Loading the RewardTrainer from TRL
trainer = RewardTrainer(
    model=model,
    args=training_args,
    tokenizer=tokenizer,
    train_dataset=formatted_dataset["train"],
    eval_dataset=formatted_dataset["test"],
)
trainer.train()

这段代码是用于训练奖励模型（reward model）的Python脚本

model_name = "distilroberta-base":
# 设置要训练的基础模型名称为distilroberta-base，这是一个预训练的模型，适用于序列分类任务。

model = AutoModelForSequenceClassification.from_pretrained(model_name, num_labels=1):
# 加载预训练的模型，并指定这是一个单标签分类任务。

tokenizer = AutoTokenizer.from_pretrained(model_name):
# 加载与模型相对应的分词器。

if tokenizer.pad_token is None: tokenizer.pad_token = tokenizer.eos_token; model.config.pad_token_id = model.config.eos_token_id:
# 如果分词器没有指定填充（padding）标记，则将其设置为结束（end-of-sequence）标记，并更新模型配置以匹配。

def formatting_func(examples): ...:
# 定义一个函数formatting_func，该函数用于格式化输入数据，使其适合模型训练。

formatted_dataset = prepared_dataset.map(formatting_func):
# 使用formatting_func函数处理prepared_dataset数据集，将其转换为模型训练所需的格式。

formatted_dataset = formatted_dataset.train_test_split():
# 将格式化后的数据集分割为训练集和测试集。

training_args = TrainingArguments(...):
# 配置训练参数，包括输出目录、每个设备的训练批次大小、评估策略、日志记录步骤、训练轮数等。

trainer = RewardTrainer(...):
# 从TRL（Training with Rewards Library）加载RewardTrainer，用于奖励模型的训练。

trainer.train():
# 启动训练过程。

以上代码首先加载了一个预训练的模型和相应的分词器，然后定义了一个数据格式化函数，该函数将指令和选择的答案或拒绝的答案组合起来，并使用分词器进行编码。接着，它将数据集映射到这个格式化函数上，并将其分割为训练集和测试集。然后，它设置了训练参数，并使用RewardTrainer来训练模型。然后，调用trainer.train()来开始训练过程。

这是一个能够评估答案质量的模型，其中选择的答案和拒绝的答案将被用来训练模型识别高质量和低质量的答案。

官网提供的日志记录：

Some weights of the model checkpoint at distilroberta-base were not used when initializing RobertaForSequenceClassification: ['lm_head.bias', 'roberta.pooler.dense.bias', 'lm_head.layer_norm.bias', 'roberta.pooler.dense.weight', 'lm_head.dense.weight', 'lm_head.decoder.weight', 'lm_head.dense.bias', 'lm_head.layer_norm.weight']
- This IS expected if you are initializing RobertaForSequenceClassification from the checkpoint of a model trained on another task or with another architecture (e.g. initializing a BertForSequenceClassification model from a BertForPreTraining model).
- This IS NOT expected if you are initializing RobertaForSequenceClassification from the checkpoint of a model that you expect to be exactly identical (initializing a BertForSequenceClassification model from a BertForSequenceClassification model).
Some weights of RobertaForSequenceClassification were not initialized from the model checkpoint at distilroberta-base and are newly initialized: ['classifier.dense.bias', 'classifier.out_proj.bias', 'classifier.out_proj.weight', 'classifier.dense.weight']
You should probably TRAIN this model on a down-stream task to be able to use it for predictions and inference.
Map:   0%|          | 0/9 [00:00<?, ? examples/s]
You're using a RobertaTokenizerFast tokenizer. Please note that with a fast tokenizer, using the `__call__` method is faster than using a method to encode the text followed by a call to the `pad` method to get a padded encoding.
Could not estimate the number of tokens of the input, floating-point operations will not be computed
[10/10 00:03, Epoch 10/10]

trl/trainer/reward_trainer.py源代码

 # Copyright 2023 The HuggingFace Team. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import inspect
import warnings
from collections import defaultdict
from dataclasses import FrozenInstanceError, replace
from typing import Any, Callable, Dict, List, Optional, Tuple, Union

import pandas as pd
import torch
import torch.nn as nn
from accelerate.utils import gather_object
from datasets import Dataset
from transformers import DataCollator, PreTrainedModel, PreTrainedTokenizerBase, Trainer, TrainingArguments
from transformers.trainer_callback import TrainerCallback
from transformers.trainer_pt_utils import nested_detach
from transformers.trainer_utils import EvalPrediction

from ..import_utils import is_peft_available
from .reward_config import RewardConfig
from .utils import RewardDataCollatorWithPadding, compute_accuracy, print_rich_table


if is_peft_available():
    from peft import PeftModel, get_peft_model, prepare_model_for_kbit_training


class RewardTrainer(Trainer):
    r"""
    The RewardTrainer can be used to train your custom Reward Model. It is a subclass of the
    `transformers.Trainer` class and inherits all of its attributes and methods. It is recommended to use
    an `AutoModelForSequenceClassification` as the reward model. The reward model should be trained on a dataset
    of paired examples, where each example is a tuple of two sequences. The reward model should be trained to
    predict which example in the pair is more relevant to the task at hand.

    The reward trainer expects a very specific format for the dataset. The dataset should contain two 4 entries at least
    if you don't use the default `RewardDataCollatorWithPadding` data collator. The entries should be named
    - `input_ids_chosen`
    - `attention_mask_chosen`
    - `input_ids_rejected`
    - `attention_mask_rejected`

    Optionally, you can also pass a `margin` entry to the dataset. This entry should contain the margin used to modulate the
    loss of the reward model as outlined in https://ai.meta.com/research/publications/llama-2-open-foundation-and-fine-tuned-chat-models/.
    If you don't pass a margin, no margin will be used.
    """

    _tag_names = ["trl", "reward-trainer"]

    def __init__(
        self,
        model: Optional[Union[PreTrainedModel, nn.Module]] = None,
        args: Optional[RewardConfig] = None,
        data_collator: Optional[DataCollator] = None,
        train_dataset: Optional[Dataset] = None,
        eval_dataset: Optional[Union[Dataset, Dict[str, Dataset]]] = None,
        tokenizer: Optional[PreTrainedTokenizerBase] = None,
        model_init: Optional[Callable[[], PreTrainedModel]] = None,
        compute_metrics: Optional[Callable[[EvalPrediction], Dict]] = None,
        callbacks: Optional[List[TrainerCallback]] = None,
        optimizers: Tuple[torch.optim.Optimizer, torch.optim.lr_scheduler.LambdaLR] = (
            None,
            None,
        ),
        preprocess_logits_for_metrics: Optional[Callable[[torch.Tensor, torch.Tensor], torch.Tensor]] = None,
        max_length: Optional[int] = None,
        peft_config: Optional[Dict] = None,
    ):
        """
        Initialize RewardTrainer.

        Args:
            model (`transformers.PreTrainedModel`):
                The model to train, preferably an `AutoModelForSequenceClassification`.
            args (`RewardConfig`):
                The arguments to use for training.
            data_collator (`transformers.DataCollator`):
                The data collator to use for training. If None is specified, the default data collator (`RewardDataCollatorWithPadding`) will be used
                which will pad the sequences to the maximum length of the sequences in the batch, given a dataset of paired sequences.
            train_dataset (`datasets.Dataset`):
                The dataset to use for training.
            eval_dataset (`datasets.Dataset`):
                The dataset to use for evaluation.
            tokenizer (`transformers.PreTrainedTokenizerBase`):
                The tokenizer to use for training. This argument is required if you want to use the default data collator.
            model_init (`Callable[[], transformers.PreTrainedModel]`):
                The model initializer to use for training. If None is specified, the default model initializer will be used.
            compute_metrics (`Callable[[transformers.EvalPrediction], Dict]`, *optional* defaults to `compute_accuracy`):
                The metrics to use for evaluation. If no metrics are specified, the default metric (`compute_accuracy`) will be used.
            callbacks (`List[transformers.TrainerCallback]`):
                The callbacks to use for training.
            optimizers (`Tuple[torch.optim.Optimizer, torch.optim.lr_scheduler.LambdaLR]`):
                The optimizer and scheduler to use for training.
            preprocess_logits_for_metrics (`Callable[[torch.Tensor, torch.Tensor], torch.Tensor]`):
                The function to use to preprocess the logits before computing the metrics.
            max_length (`int`, defaults to `None`):
                The maximum length of the sequences in the batch. This argument is required if you want to use the default data collator.
            peft_config (`Dict`, defaults to `None`):
                The PEFT configuration to use for training. If you pass a PEFT configuration, the model will be wrapped in a PEFT model.
        """
        if type(args) == TrainingArguments:
            warnings.warn(
                "Using `transformers.TrainingArguments` for `args` is deprecated and will be removed in a future version. Please use `RewardConfig` instead.",
                FutureWarning,
            )
            if max_length is not None:
                warnings.warn(
                    "The `max_length` argument is deprecated and will be removed in a future version. Please use the `RewardConfig` to set `max_length` instead.",
                    FutureWarning,
                )
        else:
            if max_length is not None and args.max_length is not None:
                raise ValueError(
                    "You cannot specify both `max_length` and `args.max_length`. Please use the `RewardConfig` to set `max_length` once."
                )
            if max_length is not None and args.max_length is None:
                warnings.warn(
                    "The `max_length` argument is deprecated and will be removed in a future version. Please use the `RewardConfig` to set `max_length` instead.",
                    FutureWarning,
                )
        if not is_peft_available() and peft_config is not None:
            raise ValueError(
                "PEFT is not installed and you passed a `peft_config` in the trainer's kwargs, please install it to use the PEFT models"
            )
        elif is_peft_available() and peft_config is not None:
            if not isinstance(model, PeftModel):
                if getattr(model, "is_loaded_in_8bit", False) or getattr(model, "is_quantized", False):
                    _supports_gc_kwargs = "gradient_checkpointing_kwargs" in list(
                        inspect.signature(prepare_model_for_kbit_training).parameters
                    )

                    prepare_model_kwargs = {"use_gradient_checkpointing": args.gradient_checkpointing}

                    if not _supports_gc_kwargs and args.gradient_checkpointing_kwargs is not None:
                        warnings.warn(
                            "You passed `gradient_checkpointing_kwargs` in the trainer's kwargs, but your peft version does not support it. "
                            "please update to the latest version of peft to use `gradient_checkpointing_kwargs`."
                        )
                    elif _supports_gc_kwargs and args.gradient_checkpointing_kwargs is not None:
                        prepare_model_kwargs["gradient_checkpointing_kwargs"] = args.gradient_checkpointing_kwargs

                    model = prepare_model_for_kbit_training(model, **prepare_model_kwargs)

                model = get_peft_model(model, peft_config)

        if compute_metrics is None:
            compute_metrics = compute_accuracy

        if data_collator is None:
            if tokenizer is None:
                raise ValueError(
                    "max_length or a tokenizer must be specified when using the default RewardDataCollatorWithPadding"
                )
            if type(args) == TrainingArguments:
                if max_length is None:
                    warnings.warn(
                        "When using RewardDataCollatorWithPadding, you should set `max_length` in RewardConfig."
                        " It will be set to `512` by default, but you should do it yourself in the future.",
                        UserWarning,
                    )
                    max_length = 512
            else:
                if max_length is None and args.max_length is None:
                    warnings.warn(
                        "When using RewardDataCollatorWithPadding, you should set `max_length` in RewardConfig."
                        " It will be set to `512` by default, but you should do it yourself in the future.",
                        UserWarning,
                    )
                    max_length = 512
                if max_length is None and args.max_length is not None:
                    max_length = args.max_length

            data_collator = RewardDataCollatorWithPadding(tokenizer, max_length=max_length)

            if args.remove_unused_columns:
                try:  # for bc before https://github.com/huggingface/transformers/pull/25435
                    args.remove_unused_columns = False
                except FrozenInstanceError:
                    args = replace(args, remove_unused_columns=False)
                # warn users
                warnings.warn(
                    "When using RewardDataCollatorWithPadding, you should set `remove_unused_columns=False` in your RewardConfig"
                    " we have set it for you, but you should do it yourself in the future.",
                    UserWarning,
                )

            self.use_reward_data_collator = True
        else:
            self.use_reward_data_collator = False
        super().__init__(
            model=model,
            args=args,
            data_collator=data_collator,
            train_dataset=train_dataset,
            eval_dataset=eval_dataset,
            tokenizer=tokenizer,
            model_init=model_init,
            compute_metrics=compute_metrics,
            callbacks=callbacks,
            optimizers=optimizers,
            preprocess_logits_for_metrics=preprocess_logits_for_metrics,
        )

        # Add tags for models that have been loaded with the correct transformers version
        if hasattr(self.model, "add_model_tags"):
            self.model.add_model_tags(self._tag_names)

    def compute_loss(
        self,
        model: Union[PreTrainedModel, nn.Module],
        inputs: Dict[str, Union[torch.Tensor, Any]],
        return_outputs=False,
    ) -> Union[torch.Tensor, Tuple[torch.Tensor, Dict[str, torch.Tensor]]]:
        if not self.use_reward_data_collator:
            warnings.warn(
                "The current compute_loss is implemented for RewardDataCollatorWithPadding,"
                " if you are using a custom data collator make sure you know what you are doing or"
                " implement your own compute_loss method."
            )
        rewards_chosen = model(
            input_ids=inputs["input_ids_chosen"],
            attention_mask=inputs["attention_mask_chosen"],
            return_dict=True,
        )["logits"]
        rewards_rejected = model(
            input_ids=inputs["input_ids_rejected"],
            attention_mask=inputs["attention_mask_rejected"],
            return_dict=True,
        )["logits"]
        # calculate loss, optionally modulate with margin
        if "margin" in inputs:
            loss = -nn.functional.logsigmoid(rewards_chosen - rewards_rejected - inputs["margin"]).mean()
        else:
            loss = -nn.functional.logsigmoid(rewards_chosen - rewards_rejected).mean()

        if return_outputs:
            return loss, {
                "rewards_chosen": rewards_chosen,
                "rewards_rejected": rewards_rejected,
            }
        return loss

    def prediction_step(
        self,
        model: Union[PreTrainedModel, nn.Module],
        inputs: Dict[str, Union[torch.Tensor, Any]],
        prediction_loss_only: bool,
        ignore_keys: Optional[List[str]] = None,
    ) -> Tuple[Optional[torch.Tensor], Optional[torch.Tensor], Optional[torch.Tensor]]:
        inputs = self._prepare_inputs(inputs)
        if ignore_keys is None:
            if hasattr(self.model, "config"):
                ignore_keys = getattr(self.model.config, "keys_to_ignore_at_inference", [])
            else:
                ignore_keys = []

        with torch.no_grad():
            loss, logits_dict = self.compute_loss(model, inputs, return_outputs=True)

        if prediction_loss_only:
            return (loss, None, None)

        loss = loss.detach()
        logits = tuple(v for k, v in logits_dict.items() if k not in ignore_keys)
        logits = nested_detach(logits)
        # Stack accepted against rejected, mean over logits
        # and softmax to get preferences between accepted and rejected to sum to 1
        logits = torch.stack(logits).mean(dim=2).softmax(dim=0).T

        labels = torch.zeros(logits.shape[0])
        labels = self._prepare_inputs(labels)

        return loss, logits, labels

    def evaluate(self, *args, **kwargs):
        num_print_samples = kwargs.pop("num_print_samples", 4)
        self.visualize_samples(num_print_samples)
        return super().evaluate(*args, **kwargs)

    def visualize_samples(self, num_print_samples: int):
        """
        Visualize the reward model logits prediction

        Args:
            num_print_samples (`int`, defaults to `4`):
                The number of samples to print. Set to `-1` to print all samples.
        """
        eval_dataloader = self.get_eval_dataloader()
        table = defaultdict(list)
        for _, inputs in enumerate(eval_dataloader):
            _, logits, _ = self.prediction_step(self.model, inputs, prediction_loss_only=False)
            chosen_text = self.tokenizer.batch_decode(inputs["input_ids_chosen"], skip_special_tokens=True)
            rejected_text = self.tokenizer.batch_decode(inputs["input_ids_rejected"], skip_special_tokens=True)
            table["chosen_text"].extend(gather_object(chosen_text))
            table["rejected_text"].extend(gather_object(rejected_text))
            table["logits"].extend(
                gather_object([[round(inner_item, 4) for inner_item in item] for item in logits.tolist()])
            )
            if num_print_samples >= 0 and len(table["chosen_text"]) >= num_print_samples:
                break
        df = pd.DataFrame(table)
        print_rich_table(pd.DataFrame(table))
        if self.accelerator.process_index == 0:
            print_rich_table(df[:num_print_samples])
            if "wandb" in self.args.report_to:
                import wandb

                if wandb.run is not None:
                    wandb.log({"completions": wandb.Table(dataframe=df)})