Add linalg.lu
This PR modifies `lu_unpack` by:
- Using less memory when unpacking `L` and `U`
- Fuse the subtraction by `-1` with `unpack_pivots_stub`
- Define tensors of the correct types to avoid copies
- Port `lu_unpack` to be a strucutred kernel so that its `_out` version
does not incur on extra copies
Then we implement `linalg.lu` as a structured kernel, as we want to
compute its derivative manually. We do so because composing the
derivatives of `torch.lu_factor` and `torch.lu_unpack` would be less efficient.
This new function and `lu_unpack` comes with all the things it can come:
forward and backward ad, decent docs, correctness tests, OpInfo, complex support,
support for metatensors and support for vmap and vmap over the gradients.
I really hope we don't continue adding more features.
This PR also avoids saving some of the tensors that were previously
saved unnecessarily for the backward in `lu_factor_ex_backward` and
`lu_backward` and does some other general improvements here and there
to the forward and backward AD formulae of other related functions.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67833
Approved by: https://github.com/IvanYashchuk, https://github.com/nikitaved, https://github.com/mruberry
