fix eigh backward test case

[Courtesy-Xs]

Eigh API反向传播的时候，使用单独的W或者V对X求导都是没有意义的.

[paddle-bot[bot]]

Thanks for your contribution! Please wait for the result of CI firstly.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}