Sparse linear solve: CG/GMRES on trnsparse matrices

[scttfrdmn]

When the sparse matvec path in trnsparse matures, cg and gmres should accept sparse A directly. Currently both treat A as dense (torch.Tensor) or as a callable matvec.

Scope

• Extend the A argument type to Union[torch.Tensor, Callable, trnsparse.SparseMatrix] (or whatever the trnsparse sparse type is called at the time)
• Dispatch to trnsparse.spmv for the matvec path
• Add sparse-A tests

Depends on

• trnsparse providing a stable public matvec API (cross-project)