Add VAS to TRL

[idanshen]

This PR introduces the implementation of Value Augmented Sampling (VAS), based on the paper Value Augmented Sampling for Language Model Alignment and Personalization. VAS enhances language model generation by incorporating a value function that guides the decoding process.

Key Components:

• VASTrainer: Implements the training algorithm for the Value function.
• VASSampler: Implements the generation process according to the VAS decoding scheme. Uses the trained Value function to adjust the sampling probabilities during inference.