Backward operation of torch.eig for real eigenvalues (#33090)
Summary:
Another pull request to follow up issue https://github.com/pytorch/pytorch/issues/32531.
Here I implemented the backward operation for `torch.eig` with a condition that all the eigenvalues are real.
This pull request is independent of my another pull request https://github.com/pytorch/pytorch/issues/32932, which means that there is no dependency between this PR and my another PR.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33090
Differential Revision: D19814347
Pulled By: albanD
fbshipit-source-id: 2fae30964e97987abb690544df8240aedeae56e8
