SparseInverseSubset.jl dependency causes issues on non-GPL Julia builds

[ElOceanografo]

ChainRules depends on SparseInverseSubset.jl to be able to differentiate through determinants of sparse matrices. However, this requires the CHOLMOD type for sparse Cholesky decompositions from SuiteSparse.jl, which is unavailable on non-GPL Julia builds, causing errors. See here and here.

I think the cleanest way to handle this would be to add conditional statements for importing SparseInverseSubset.jl, and for the rrules for logdet and logabsdet of sparse matrices so that these are omitted if Base.USE_GPL_LIBS == false. If that sound like the right approach I will make a PR.

[oxinabox]

I think that does indeed sound right
wrap it in top level

if Base.USE_GPL_LIBS  # don't define these if we don't have CHOLMOD from SuiteSparse.jl
    using SparseInveserSubset
    function rrule(...)
    ...
end

that way we will get the usual behavours as if the rrule had not being defined -- because it wouldn't have been defined.