More stable and faster implementation of the gradient of torch.linalg.eigh (#55049)
Summary:
This PR:
- Renames symeig_backward to eigh_backward
- Improves the stability and speed of the gradient computation by doing `V(A + B)Vh` instead of `VAVh + VBVh` when both the gradients of the eigenvectors and eigenvalues are defined.
- Updates the comments of the function to make them arguably clearer
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55049
Reviewed By: ngimel
Differential Revision: D28396823
Pulled By: mruberry
fbshipit-source-id: a144482bfb1054e281b58ae1fe3cf1015bab505d
