Save the ColBERT encodings to disk.

I have a use case where I run ColBERT on CPU on a couple thousand documents. For this I don't use PLAID but the `encode` and `search_encoded_docs` methods and the search works fast enough, the problem is that encoding all these documents on CPU takes time and I don't want to encode everything everytime I deploy the model so I developed a way for saving and loading these encodings:

https://github.com/ChatFAQ/ChatFAQ/blob/cc19e4b85198062888d6320e59276db31461f4e9/chat_rag/chat_rag/retrievers/colbert_retriever.py#L163

If interested I could improve and integrate this into the `RAGPretrainedModel` or `ColBERT` classes and make a PR.

Start by reviewing the referenced chat_rag/retrievers/colbert_retriever.py implementation around line 163, then inspect the encode and search_encoded_docs methods in RAGPretrainedModel or ColBERT. The issue is complete when encodings can be saved and loaded for later searches without re-encoding the documents.

Written by the indexing model from the issue text.