Update internal code for torch.linalg.solve
Replace linalg_solve_helper with `lu_stub` + `lu_solve_stub`.
Once `lu_stub` and `lu_solve_stub` have cuSOLVER-based codepath,
`torch.linalg.solve` will have it as well.
ghstack-source-id: a5f501df96748e68fe3cc971848b8ffc0c889706
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56613
