In this PR, a `val_generations_to_log_to_swanlab` parameter has been
added. When this parameter is set to 1, it supports logging the
generated text from eval in SwanLab.

@hiyouga 

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This pull request introduces logging of validation generations to
Swanlab in addition to Wandb. The changes include updates to several
configuration files and the addition of a new logging method in the
`ray_trainer.py` file.

Key changes include:

### Configuration Updates:
* Added `val_generations_to_log_to_swanlab` parameter to the `trainer`
section in the following configuration files:
  * `examples/split_placement/config/ppo_trainer_split.yaml`
  * `verl/trainer/config/ppo_megatron_trainer.yaml`
  * `verl/trainer/config/ppo_trainer.yaml`

### Code Updates:
* Added a new method `_maybe_log_val_generations_to_swanlab` to log
validation samples to Swanlab in `verl/trainer/ppo/ray_trainer.py`
* Updated the `_validate` method to call the new Swanlab logging method
in `verl/trainer/ppo/ray_trainer.py`

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