### Checklist Before Starting

- [x] Search for similar PR(s).

### What does this PR do?

> Add support for [PF-PPO](https://arxiv.org/abs/2409.06957) in verl.

### Specific Changes

> `verl/trainer/config/ppo_trainer.yaml`: Add config for PF-PPO
`verl/trainer/ppo/core_algos.py`: Add `compute_pf_ppo_reweight_data`
function.
`verl/trainer/ppo/ray_trainer.py`: Do PF-PPO in `compute_advantage` when
`config.algorithm.use_pf_ppo` is `True`
`README.md`: Update PF-PPO in README

### Usage Example

```bash
set -x

python3 -m verl.trainer.main_ppo \
    algorithm.adv_estimator=gae \
    algorithm.use_pf_ppo=True \
    algorithm.pf_ppo.reweight_method=pow \
    algorithm.pf_ppo.weight_pow=2.0 \
    data.train_files=$HOME/data/gsm8k/train.parquet \
    data.val_files=$HOME/data/gsm8k/test.parquet \
    data.train_batch_size=1024 \
    data.max_prompt_length=512 \
    data.max_response_length=512 \
    data.filter_overlong_prompts=True \
    data.truncation='error' \
    actor_rollout_ref.model.path=deepseek-ai/deepseek-llm-7b-chat \
    actor_rollout_ref.actor.optim.lr=1e-6 \
    actor_rollout_ref.model.use_remove_padding=True \
    actor_rollout_ref.actor.ppo_mini_batch_size=256 \
    actor_rollout_ref.actor.ppo_micro_batch_size_per_gpu=16 \
    actor_rollout_ref.actor.fsdp_config.param_offload=False \
    actor_rollout_ref.actor.fsdp_config.optimizer_offload=False \
    actor_rollout_ref.actor.use_kl_loss=False \
    actor_rollout_ref.model.enable_gradient_checkpointing=True \
    actor_rollout_ref.rollout.log_prob_micro_batch_size_per_gpu=32 \
    actor_rollout_ref.rollout.tensor_model_parallel_size=4 \
    actor_rollout_ref.rollout.name=vllm \
    actor_rollout_ref.rollout.gpu_memory_utilization=0.4 \
    actor_rollout_ref.rollout.n=5 \
    critic.optim.lr=1e-5 \
    critic.model.use_remove_padding=True \
    critic.model.path=deepseek-ai/deepseek-llm-7b-chat \
    critic.model.enable_gradient_checkpointing=True \
    critic.ppo_micro_batch_size_per_gpu=32 \
    critic.model.fsdp_config.param_offload=False \
    critic.model.fsdp_config.optimizer_offload=False \
    algorithm.use_kl_in_reward=False \
    trainer.critic_warmup=0 \
    trainer.logger=['console','wandb'] \
    trainer.project_name='verl_example_gsm8k' \
    trainer.experiment_name='deepseek_llm_7b_function_rm' \
    trainer.n_gpus_per_node=8 \
    trainer.nnodes=1 \
    trainer.save_freq=20 \
    trainer.test_freq=1 \
    trainer.total_epochs=15 $@
```

### Test

Simple gsm8k test.

<img width="502" alt="image"
src="https://github.com/user-attachments/assets/4298ce20-a691-4edb-8e4a-ef68fb0fb6be"
/>

### Checklist Before Submitting

- [x] Read the [Contribute
Guide](https://github.com/volcengine/verl?tab=readme-ov-file#contribution-guide).
- [x] Apply [pre-commit
checks](https://github.com/volcengine/verl?tab=readme-ov-file#code-linting-and-formatting).
- [x] Add `[BREAKING]` to the PR title if it breaks any API.
- [x] Update the documentation about your changes in the
[docs](https://github.com/volcengine/verl/tree/main/docs).
- [x] Add CI test(s) if necessary.

---------

Co-authored-by: hoshi-hiyouga <hiyouga@buaa.edu.cn>