custom reward function support for ppo trainer

[August-murr]

What does this PR do?

Fixes #2518

Adding support for a custom reward function for the PPO trainer.

How it works

Write a custom function that takes a list of texts as input, representing a batch of responses, and outputs a list of scores.

def custom_reward_function(texts: list) -> list:
    """
    Custom reward function that applies a given reward logic to each item in the list.
    
    Args:
        items (list): List of items to evaluate.
    
    Returns:
        list: List of rewards based on the provided reward logic.
    """
    rewards = [reward_logic(item) for item in items]

I will add more documentation and explanations later after running several tests to make sure the implementation is functional.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a GitHub issue? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines.
• Did you write any new necessary tests?

Who can review?

@qgallouedec

[August-murr]

This is where the primary change are:
modifying the get_reward function to work with both a nn.Module and a Callable.

[HuggingFaceDocBuilderDev]

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