Add top_k Support for Tree Collapsing Functionality

[parthsarthi03]

This PR introduces the top_k parameter to the retrieve_information_collapse_tree method in the TreeRetriever class. It allows users to control the number of top nodes to consider when retrieving information from the collapsed tree.

Usage:

• The top_k parameter is an integer that specifies the number of top nodes to consider during retrieval.
• It defaults to 10 if not provided, meaning the method will consider the top 10 most relevant nodes.
• Users can adjust the value of top_k to retrieve more or fewer nodes based on their requirements.

Functionality:

• The retrieve_information_collapse_tree method retrieves the most relevant information from the collapsed tree based on the given query.
• It uses the max_tokens parameter to limit the maximum number of tokens in the retrieved context, defaulting to 3500 tokens.
• The method calculates the distances between the query embedding and the node embeddings to determine the most relevant nodes.
• It then iterates over the top top_k nodes, adding them to the selected nodes list until the token limit is reached.
• it returns the selected nodes and the retrieved context.

[salman-abdullah]

Looks good to me!