[Feature]: Unify and Relocate the Index and Search Module to the Retrieval Layer

[jzhoubu]

1. Integration of dense and sparse pipeline

Combine the dense and sparse pipelines (indexing, searching, evaluation) into a single unified pipeline within retriever module. Currently, the only difference between sparse and desne pipelines is the type of the index and search function, which should be determined when a specific retrieval architecture is chosen. In the future, I will prioritize sparse functions, with dense functions only serving as references.

2. Reorganization of index and search modules

Will relocate index and search modules into the retrieval module to for easy-usability. These components are currently located in the example directory.

3. Removal of replication parts for academic

Future academic replication efforts will be relocated to a separate repository for clarity in the main project.