Add Support for Gemma 2 Models

[shrijayan]

This pull request introduces support for Gemma 2 models in the llm2vec repository by implementing the following enhancements:

Key Changes:

1. Updated Transformers Version Constraint:

   • Modified dependencies to allow transformers==4.45.2, ensuring compatibility with the latest Gemma 2 configurations.

2. Added Support for Gemma2Config:

   • Integrated Gemma2Config to enable seamless usage of Gemma 2 models within the llm2vec framework.
   • Ensured backward compatibility with existing model configurations.

3. Enhanced GemmaBiModel and GemmaBiForMNTP:

   • Updated GemmaBiModel and GemmaBiForMNTP to handle Gemma 2 architecture properly.
   • Refactored encoding logic to leverage new features from Gemma 2.