Load docs and create index with instantiation of reranker instead of with each rerank.

Mostly thinking about this in the context of making the colbert reranker more useful... it would be great to be able to load the docs and create the index when you instantiate the reranker. This prevents repeating that computation each time we wish to produce a result.

I am happy to take this up, but wanted to raise it as an issue and see if it fits within the spirit of the project.

I do think that extending the colbert ranker (and others) in the way I describe will allow a broader use case.