Fix: Update imports to resolve ModuleNotFoundError for 'langchain.docstore'

[Yugsolanki]

Description

This pull request addresses the ModuleNotFoundError: No module named 'langchain.docstore' error that was occurring in the BaseRetriever class. The imports have been reviewed and updated to ensure compatibility with the current structure of the langchain library.

Changes Made

• Updated import statements to reflect the correct modules from langchain.
• Ensured that the code adheres to the latest version of the langchain library.

Related Issues

• Resolves #4765

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