Simplify and optimize linalg.solve
This PR heavily simplifies the code of `linalg.solve`. At the same time,
this implementation saves quite a few copies of the input data in some
cases (e.g. A is contiguous)
We also implement it in such a way that the derivative goes from
computing two LU decompositions and two LU solves to no LU
decompositions and one LU solves. It also avoids a number of unnecessary
copies the derivative was unnecessarily performing (at least the copy of
two matrices).
On top of this, we add a `left` kw-only arg that allows the user to
solve `XA = B` rather concisely.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74046
Approved by: https://github.com/nikitaved, https://github.com/IvanYashchuk, https://github.com/mruberry
