Updated derivative rules for complex QR decomposition (#48489)
Summary:
Updated `qr_backward` to work correctly for complex-valued inputs.
Added `torch.qr` to list of complex tests.
The previous implementation for real-valued differentiation used equation 42 from https://arxiv.org/abs/1001.1654
The current implementation is a bit simpler but the result for the real-valued input case is the same and all tests still pass.
Derivation of complex-valued QR differentiation https://giggleliu.github.io/2019/04/02/einsumbp.html
Ref. https://github.com/pytorch/pytorch/issues/33152
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48489
Reviewed By: bdhirsh
Differential Revision: D25272344
Pulled By: albanD
fbshipit-source-id: b53c1fca1683f4aee5f4d5ce3cab9e559170e7cf
