In the code example for topic modeling with Gemini, what is the purpose of the `block_size` parameter in the `assign_topics` function, and how does it affect the processing of posts?

block_size determines how many social media posts are grouped into a single batch for topic assignment. The assign_topics function loops through the posts in steps of block_size, builds a sub-dataframe for each batch, and sends that batch to the Gemini API for analysis. This chunking lets the code process large datasets efficiently by handling one manageable block of posts at a time.