Simplify forward / backward AD for linalg.eigh and add checks (#70528)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70528
This PR adds checks for the backward of `linalg.eigh`, similar to those
deduced in https://github.com/pytorch/pytorch/pull/70253
It also makes its the implementation parallel that of the (fwd/bwd) derivative of
`torch.linalg.eig` and it makes most OpInfo tests pass.
cc jianyuh nikitaved pearu mruberry walterddr IvanYashchuk xwang233 Lezcano
Test Plan: Imported from OSS
Reviewed By: mruberry
Differential Revision: D33530149
Pulled By: albanD
fbshipit-source-id: 1f368b8d450d4e9e8ae74d3881c78513c27eb956
