ATen lu_unpack. Required for making `torch.lu_solve` differentiable. (#46913)
Summary:
Backward methods for `torch.lu` and `torch.lu_solve` require the `torch.lu_unpack` method.
However, while `torch.lu` is a Python wrapper over a native function, so its gradient is implemented via `autograd.Function`,
`torch.lu_solve` is a native function, so it cannot access `torch.lu_unpack` as it is implemented in Python.
Hence this PR presents a native (ATen) `lu_unpack` version. It is also possible to update the gradients for `torch.lu` so that backward+JIT is supported (no JIT for `autograd.Function`) with this function.
~~The interface for this method is different from the original `torch.lu_unpack`, so it is decided to keep it hidden.~~
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46913
Reviewed By: albanD
Differential Revision: D28355725
Pulled By: mruberry
fbshipit-source-id: 281260f3b6e93c15b08b2ba66d5a221314b00e78
