Census cell similarity search: experimental Python API for searching given AnnData

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Having built the vector indexes on Census embeddings (#694 chanzuckerberg/cellxgene-census#1113), develop the Python API inside cellxgene_census.experimental to input an AnnData and identify the most-similar Census cells.

This necessitates running a forward pass of the embedding model (starting with scVI) on the given AnnData. In case that causes a lot of complications, we can create an initial demo that searches existing Census cells against the index and break out the forward passes into a separate issue. It might even end up involving a docker image or web service of some sort.