Update internal code for torch.linalg.solve (#56613)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56613
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.
Test Plan: Imported from OSS
Reviewed By: zou3519
Differential Revision: D28627408
Pulled By: mruberry
fbshipit-source-id: b95bbdf35f845a56a1489c04b53742a01b36e789
