Add linalg.lu_factor (#66933)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66933
This PR exposes `torch.lu` as `torch.linalg.lu_factor` and
`torch.linalg.lu_factor_ex`.
This PR also adds support for matrices with zero elements both in
the size of the matrix and the batch. Note that this function simply
returns empty tensors of the correct size in this case.
We add a test and an OpInfo for the new function.
This PR also adds documentation for this new function in line of
the documentation in the rest of `torch.linalg`.
Fixes https://github.com/pytorch/pytorch/issues/56590
Fixes https://github.com/pytorch/pytorch/issues/64014
cc jianyuh nikitaved pearu mruberry walterddr IvanYashchuk xwang233 Lezcano
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D32521980
Pulled By: mruberry
fbshipit-source-id: 26a49ebd87f8a41472f8cd4e9de4ddfb7f5581fb
