eig_backward: faster and with complex support (#52875)
Summary:
As per title. Compared to the previous version, it is lighter on the usage of `at::solve` and `at::matmul` methods.
Fixes https://github.com/pytorch/pytorch/issues/51621
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52875
Reviewed By: mrshenli
Differential Revision: D26768653
Pulled By: anjali411
fbshipit-source-id: aab141968d02587440128003203fed4b94c4c655
