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: agolynski
Differential Revision: D28379394
Pulled By: mruberry
fbshipit-source-id: b47f66bc1ee12715da11dcffc92e31e67fa8c8f6
