fix(numeric type issue): 🐛 making sure the division returns an int

[Almonok]

Context

While working with RAGatouille (using colbert-ai version 0.2.19) to build and subsequently update an index, I encountered an error related to the persist_to_disk method in the IndexUpdater class. The error message was: TypeError: slice indices must be integers or None or have an __index__ method.

Analysis

Upon investigation, I identified a potential issue with the calculation of avg_chunksize. The value was being computed as a float, which later caused an error when attempting to use it for slicing, specifically in the calculation of pid_end.

To resolve this, I adjusted the computation to ensure that avg_chunksize is an integer. However, I have some reservations about whether this change aligns with the original algorithm's intent, given that using a // b results in the floor of the division.

Thank you for maintaining this repository !