Update the docs of torch.eig about derivative (#47598)
Summary:
Related: https://github.com/pytorch/pytorch/issues/33090
I just realized that I haven't updated the docs of `torch.eig` when implementing the backward.
Here's the PR updating the docs about the grad of `torch.eig`.
cc albanD
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47598
Reviewed By: heitorschueroff
Differential Revision: D24829373
Pulled By: albanD
fbshipit-source-id: 89963ce66b2933e6c34e2efc93ad0f2c3dd28c68
