Fix and improve the efficiency of the backward of xlog* functions. (#82713)
That is `xlogy`, `special.xlogy`, `special.xlog1py`.
Fixes https://github.com/pytorch/pytorch/issues/80770
Fixes https://github.com/pytorch/pytorch/issues/74279
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82713
Approved by: https://github.com/albanD
