Improve derivative of QR decomposition
We derive and implement a more concise rule for the forward and backward
derivatives of the QR decomposition. While doing this we:
- Fix the composite compliance of `linalg.qr` and we make it support batches
- Improve the performance and simplify the implementation of both foward and backward
- Avoid saving the input matrix for the backward computation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76115
Approved by: https://github.com/nikitaved, https://github.com/albanD
