Call create_collection during the first add_document call in Voyager index to infer embedding dimension

[NohTow]

Right now, we are creating the Voyager index during the initialization of the index model, which force us to have the target embedding dimension and so to ask the user to feed it.
@raphaelsty proposed to create the actual index object only when adding the first document (if index is none) so we can infer the embedding dimension, which is a way nicer UX.

[raphaelsty]

@NohTow Previously done in cherche faiss index, it's the way to go I think to initialize the tree once you start adding documents. I'll handle this MR unless you want to :).

Warning: we should not break backward compatibility and let embedding input parameter or at least include kwargs ⚠️