You have to initialize model that you want to use with Ranker function in flashrank, you can define like:

from flashrank import Ranker 

ranker = Ranker(model_name="ms-marco-MiniLM-L-12-v2")

then fill it like:

compressor = FlashrankRerank(model="ms-marco-MiniLM-L-12-v2", top_n=3)

compression_retriever = ContextualCompressionRetriever(
base_compressor=compressor, base_retriever=retriever)