[Option] Parallelize preconditioners across ranks

[luciaquirke]

More VRAM efficient variant where preconditioners can be spread across an arbitrary number of nodes to compute large outer products. This is useful because preconditioners are often applied to a query and then the query is run across a large dataset, so slow but VRAM-efficient preconditioner computation and usage is a scalable pattern.

Because the preconditioners don't necessarily fit on a single GPU we use GLOO to do distributed CPU operations.