Refacto retriever

[Ahmath-Gadji]

Changes

• Removed logger arguments and centralized logging.
• Cleaned up retriever classes by removing unused parameters like criteria.
• Updated retriever setup to take LLMs explicitly.
• Added RETRIEVER_TYPE env variable for flexible configuration.
• Unified LLM config handling across retrievers.
• Fixed RagPipeline to use the correct LLM for contextualization.
• Removed unused imports and a noisy warning in app_front.py.
• Updated all retriever and pipeline calls to match new constructor signatures.

Result

Simpler, cleaner, and more consistent retriever and pipeline setup.