ppo.py

import math
from pathlib import Path
import copy
from tqdm import tqdm
from functools import partial
from collections import deque, namedtuple
from random import randrange

from beartype import beartype
from beartype.typing import List, Optional, Callable, Deque

from einops import rearrange, repeat
from einops.layers.torch import Rearrange

import torch
from torch import nn
import torch.nn.functional as F

from torch.optim import Adam
from torch.utils.data import Dataset, DataLoader
from torch.nn.utils.rnn import pad_sequence

import pytorch_lightning as pl
from pytorch_lightning.utilities import rank_zero_info
from pytorch_lightning.strategies import DeepSpeedStrategy
from deepspeed.ops.adam import DeepSpeedCPUAdam, FusedAdam

from src.model import RWKV
from src.rlhf.reward import RewardModel
from src.rlhf.optimizer import get_optimizer
from src.rlhf.utils import masked_mean, eval_decorator

# actor critic

PPOActionCriticReturn = namedtuple('PPOActionCriticReturn', [
    'actions',
    'sequence',
    'mask',
    'prompt_mask',
    'action_logits',
    'values'
])

@beartype
class ActorCritic(nn.Module):
    def __init__(
        self,
        args,
        actor: RWKV,
        critic: RWKV,
        pooled_values = False
    ):
        super().__init__()

        self.actor = actor
        self.critic = critic

        self.pooled_values = pooled_values
        self.value_head = nn.Sequential(
            nn.Linear(args.n_embd, 1),
            Rearrange('... 1 -> ...')
        )

        nn.init.zeros_(self.value_head[0].bias)
        nn.init.orthogonal_(self.value_head[0].weight, gain = math.sqrt(2))

    @torch.no_grad()
    @eval_decorator
    def generate(
        self,
        state,
        max_seq_len,
        eos_token = None,
        return_values = False
    ):
        # 产生一条 response，相当于采取了一次 action
        actions = self.actor.generate(
            max_seq_len,
            prompt = state,       
            eos_token = eos_token
        )

        # 将 prompt (state) 和 response (action) 进行拼接
        sequence = torch.cat((state, actions), dim = -1)
        action_len = actions.shape[-1]
        state_len = state.shape[-1]

        # 构建 prompt_mask (state_mask) 和 response_mask (action_mask)
        prompt_mask = torch.arange(sequence.shape[-1], device = state.device) < state_len
        prompt_mask = repeat(prompt_mask, 'n -> b n', b = sequence.shape[0])

        action_mask = ~prompt_mask

        # 考虑 eos token
        mask = None
        if exists(eos_token):
            mask = ((sequence == eos_token).cumsum(dim = -1) == 0)
            mask = F.pad(mask, (1, -1), value = True) # include eos token
            action_mask &= mask

        # 将生成的 sequence 输入到 actor 中，得到 action_logits
        # 将生成的 sequence 输入到 critic 中，得到 value
        action_logits, value = self.forward(
            sequence,
            mask = action_mask,
            return_values = return_values
        )        

        return PPOActionCriticReturn(
            actions,
            sequence,
            mask,
            prompt_mask,
            action_logits,
            value
        )

    def forward(
        self,
        x,
        mask = None,
        return_values = True
    ):
        action_logits, _ = self.actor(
            x,
            ppo_train = True
        )

        if not return_values:
            return action_logits, None

        _, critic_embeds = self.critic(
            x,
            return_only_embedding = True,
            ppo_train = True
        )

        if self.pooled_values:
            critic_embeds = shift(critic_embeds, shift = 1, dim = -2)
            critic_embeds = masked_mean(critic_embeds, mask, dim = 1)

        values = self.value_head(critic_embeds)

        return action_logits, values

@beartype
class ExperienceDataset(Dataset):
    def __init__(
        self,
        data: List[torch.Tensor],
        device = None
    ):
        super().__init__()
        self.data = data
        self.device = device

    def __len__(self):
        return self.data[0].shape[0]

    def __getitem__(self, ind):
        return tuple(map(lambda t: t[ind].to(self.device), self.data))

def create_dataloader(data, batch_size, shuffle = True, device = None, **kwargs):
    ds = ExperienceDataset(data, device = device)
    return DataLoader(ds, batch_size = batch_size, shuffle = shuffle, **kwargs)

# helper functions

def exists(val):
    return val is not None

def default(val, d):
    return val if exists(val) else d

def masked_normalize(t, eps = 1e-5, mask = None, dim = None):
    dim = default(dim, tuple(range(t.ndim)))
    kwargs = dict(dim = dim, keepdim = True)

    mean = masked_mean(t, mask = mask, **kwargs)
    mean_centered = t - mean
    var = masked_mean(mean_centered ** 2, mask = mask, **kwargs)

    return mean_centered * var.clamp(min = eps).rsqrt()

def pad_sequence_fixed(sequences, *args, **kwargs):
    first_el = sequences[0]
    has_no_dimension = first_el.ndim == 0

    # if no dimensions, add a single dimension
    if has_no_dimension:
        sequences = tuple(map(lambda t: t[None], sequences))

    out = pad_sequence(sequences, *args, **kwargs)

    if has_no_dimension:
        out = rearrange(out, '... 1 -> ...')

    return out

def log(t, eps = 1e-20):
    return torch.log(t.clamp(min = eps))

def log_prob(prob, indices):
    assert prob.shape[:2] == indices.shape, f'preceding shapes of prob {prob.shape[:2]} and indices {indices.shape} must match'
    return log(prob.gather(-1, indices[..., None])).squeeze(-1)

def shift(t, value = 0, shift = 1, dim = -1):
    zeros = (0, 0) * (-dim - 1)
    return F.pad(t, (*zeros, shift, -shift), value = value)

def masked_entropy(prob, dim = -1, mask = None):
    entropies = (prob * log(prob)).sum(dim = -1)
    return masked_mean(entropies, mask = mask).mean()

def masked_kl_div(prob1, prob2, mask = None):
    """
    need to account for variable sequence lengths, therefore not using the built-in functional version
    """
    kl_divs = (prob1 * (log(prob2) - log(prob1))).sum(dim = -1)

    if not exists(mask):
        return kl_divs.mean()

    return masked_mean(kl_divs, mask).mean()

def clipped_value_loss(values, rewards, old_values, clip):
    value_clipped = old_values + (values - old_values).clamp(-clip, clip)
    value_loss_1 = (value_clipped.flatten() - rewards) ** 2
    value_loss_2 = (values.flatten() - rewards) ** 2
    return torch.mean(torch.max(value_loss_1, value_loss_2))

# rlhf

@beartype
class RLHF(pl.LightningModule):
    def __init__(
        self,
        args,
        actor: RWKV,
        critic: RWKV,
        reward_model: RewardModel
    ):
        super().__init__()

        self.args = args

        # 使用 RWKV 初始化 actor_critic
        actor_critic = ActorCritic(
            args=self.args,
            actor=actor,
            critic=critic,
            pooled_values = args.critic_pooled_values
        ).to(actor.device)

        self.actor_critic = actor_critic

        # 将 reward_model 设置为 evaluation 模式 
        self.reward_model = reward_model.eval()

    def save(self, filepath = './checkpoint.pt'):
        torch.save(self.actor_critic.state_dict(), filepath)

    def load(self, filepath = './checkpoint.pt'):
        state_dict = torch.load(filepath)
        self.actor_critic.load_state_dict(state_dict)
    
    def configure_optimizers(self):
        args = self.args
        if args.layerwise_lr > 0:
            lr_1x = set()
            lr_2x = set()
            lr_3x = set()
            for n, p in self.named_parameters():
                if "time_mix" in n:
                    if args.my_pile_stage == 2:
                        lr_2x.add(n)
                    else:
                        lr_1x.add(n)
                elif "time_decay" in n:
                    if args.my_pile_stage == 2:
                        lr_3x.add(n)
                    else:
                        lr_2x.add(n)
                elif "time_first" in n:
                    lr_3x.add(n)
                else:
                    lr_1x.add(n)
            lr_1x = sorted(list(lr_1x))
            lr_2x = sorted(list(lr_2x))
            lr_3x = sorted(list(lr_3x))
            param_dict = {n: p for n, p in self.named_parameters()}
            if args.my_pile_stage == 2:
                optim_groups = [
                    {"params": [param_dict[n] for n in lr_1x], "weight_decay": 0.0, "my_lr_scale": 1.0},
                    {"params": [param_dict[n] for n in lr_2x], "weight_decay": 0.0, "my_lr_scale": 5.0},# test: 2e-3 / args.lr_init},
                    {"params": [param_dict[n] for n in lr_3x], "weight_decay": 0.0, "my_lr_scale": 5.0},# test: 3e-3 / args.lr_init},
                ]
            else:
                optim_groups = [
                    {"params": [param_dict[n] for n in lr_1x], "weight_decay": 0.0, "my_lr_scale": 1.0},
                    {"params": [param_dict[n] for n in lr_2x], "weight_decay": 0.0, "my_lr_scale": 2.0},
                    {"params": [param_dict[n] for n in lr_3x], "weight_decay": 0.0, "my_lr_scale": 3.0},
                ]
        else:
            optim_groups = [
                {"params": [p for n, p in self.named_parameters()], "weight_decay": 0.0},
            ]

        if self.deepspeed_offload:
            return DeepSpeedCPUAdam(optim_groups, lr=self.args.lr_init, betas=self.args.betas, eps=self.args.adam_eps, bias_correction=True, adamw_mode=False, weight_decay=0, amsgrad=False)
        return FusedAdam(optim_groups, lr=self.args.lr_init, betas=self.args.betas, eps=self.args.adam_eps, bias_correction=True, adam_w_mode=False, weight_decay=0, amsgrad=False)
        # return ZeroOneAdam(optim_groups, lr=self.args.lr_init, betas=self.args.betas, eps=self.args.adam_eps, bias_correction=True, weight_decay=0, amsgrad=False, cuda_aware=False)

    @property
    def deepspeed_offload(self) -> bool:
        strategy = self.trainer.strategy
        if isinstance(strategy, DeepSpeedStrategy):
            cfg = strategy.config["zero_optimization"]
            return cfg.get("offload_optimizer") or cfg.get("offload_param")
        return False

    @torch.no_grad()
    def generate(
        self,
        max_seq_len,
        prompt,
        num_samples = 4  # sample 4 per prompt and select the one with highest reward
    ):
        ''' 未参与训练，仅推理时使用
        '''

        assert prompt.ndim == 1, 'only one prompt allowed at a time for now'
        prompt = repeat(prompt, 'n -> b n', b = num_samples)

        self.actor_critic.eval()
        (
            actions,
            sequences,
            mask,
            prompt_mask,
            action_logits,
            _
        ) = self.actor_critic.generate(
            prompt,
            max_seq_len = max_seq_len,
            return_values = False
        )

        rewards = self.reward_model(
            sequences,
            prompt_mask = prompt_mask,
            mask = mask,
            sample = True
        )

        best_sequence_index = rewards.topk(1, dim = -1).indices

        best_sequence = sequences[best_sequence_index]
        best_sequence = rearrange(best_sequence, '1 ... -> ...')

        return best_sequence

    def training_step(self, batch, batch_idx):
        sequences, \
        prompt_masks, \
        masks, \
        old_action_probs, \
        old_log_probs, \
        rewards, \
        old_values = batch

        # PPO training
        action_masks = ~prompt_masks & masks

        action_logits, values = self.actor_critic(
            sequences,
            mask = action_masks
        )

        action_logits = shift(action_logits, shift=1, dim=-2) # need to shift along sequence dimension by 1, since actions start from the last prompt (state) token
        action_len = old_log_probs.shape[-1]

        action_probs = action_logits.softmax(dim = -1)
        action_log_probs = log_prob(action_probs, sequences)
        action_log_probs = action_log_probs[:, -action_len:]

        # calculate entropies, taking into account which part of the sequence is actually an action

        entropies = masked_entropy(action_probs, mask = action_masks)

        # calculate kl div between old action probs and new ones, taking into account which part of the sequence is action or not

        kl_div_loss = 0.

        if self.args.kl_div_loss_weight > 0:
            kl_div_loss = masked_kl_div(action_probs, old_action_probs, mask = action_masks) * self.args.kl_div_loss_weight

        # handle non-pooled values

        normalize_kwargs = dict()

        if old_values.ndim == 2:
            old_values, values = map(lambda t: shift(t, shift = 1, dim = -2), (old_values, values))

            old_values = old_values[:, -action_len:]
            values = values[:, -action_len:]
            rewards = rearrange(rewards, 'b -> b 1')
            normalize_kwargs = dict(dim = -1, mask = action_masks[:, -action_len:])

        if values.ndim < rewards.ndim:
            values = rearrange(values, '... -> ... 1')

        # calculate clipped surrogate objective, classic PPO loss

        ratios = (action_log_probs - old_log_probs).exp()
        advantages = masked_normalize(rewards - old_values, **normalize_kwargs)

        if advantages.ndim == 1:
            advantages = rearrange(advantages, 'b -> b 1')

        surr1 = ratios * advantages
        surr2 = ratios.clamp(1 - self.args.eps_clip, 1 + self.args.eps_clip) * advantages
        policy_loss = - torch.min(surr1, surr2) - self.args.beta_s * entropies

        # actor loss (也称为 policy loss, 是最终要使用模型的 loss)
        actor_loss = policy_loss.mean() + kl_div_loss

        # critic loss (也称为 value loss)
        # update value network separate from policy network
        critic_loss = clipped_value_loss(values, rewards, old_values, self.args.value_clip)
        critic_loss = critic_loss.mean()

        return {'actor_loss': actor_loss.item(), 'critic_loss': critic_loss.item()}

    def make_experience(self, prompts, eos_token=None, temperature=1):
        ''' 通过与 environment 交互产生训练数据
        '''
        
        device = self.device

        # select a bunch of random states (prompts)
        # and get the action (sampled sequence from rwkv as well as the action probs)
        # also calculate the reward using reward model and store
        # 随机挑选一条 prompt
        rand_prompt_index = randrange(0, len(prompts))
        state = prompts[rand_prompt_index]

        # remove padding from state
        state_mask = state != self.args.pad_value
        state = state[state_mask]

        # get predicted sequence
        # 与 environment 进行交互，其中返回的：
        #   action 是 response，
        #   sequence 是 prompt + response， 
        (
            actions,
            sequence,
            mask,
            prompt_mask,
            action_logits,
            value
        ) = self.actor_critic.generate(
            rearrange(state, 'n -> 1 n'),
            max_seq_len = self.args.ctx_len,
            eos_token = eos_token,
            return_values = True
        )
        action_logits = shift(action_logits, shift = 1, dim = -2) # need to shift along sequence dimension by 1, since actions start from the last prompt (state) token

        action_prob = action_logits.softmax(dim = -1)

        action_len = actions.shape[-1]
        action_log_prob = log_prob(action_prob, sequence)
        action_log_prob = action_log_prob[:, -action_len:]

        actions = rearrange(actions, '1 ... -> ...')

        # get reward as given by supervised trained reward model
        sequence = torch.cat((state, actions), dim = 0)

        prompt_length = len(state)
        prompt_mask = torch.arange(sequence.shape[-1], device = device) < prompt_length

        sequence = rearrange(sequence, 'n -> 1 n')
        prompt_mask = rearrange(prompt_mask, 'n -> 1 n')
        mask = rearrange(mask, 'n -> 1 n') if exists(mask) else torch.ones(sequence.shape, dtype = torch.bool, device = device)

        reward = self.reward_model(
            sequence,
            prompt_mask = prompt_mask,
            mask = mask,
            sample = True
        )

        return (
            sequence,
            prompt_mask,
            mask,
            action_prob,
            action_log_prob,
            reward,
            value
        )